Interrogator and Interrogation System Employing the Same

ABSTRACT

An interrogator, methods of discerning the presence of an object, and interrogation systems employing the same. In one embodiment, the interrogation systems include multiple interrogators that communicate with a base command unit to track a location of an object. In another embodiment wherein the object is an RFID object (e.g., an object with an RFID tag), the interrogators employ signal processing techniques such as precharging the RFID object, and correlating a reference code with a reply code from the RIFD object using selected techniques to increase a sensitivity of the interrogator, especially for adverse environments. In other embodiments, the interrogation systems include variations of metal instruments and sponges employed therewith. In yet another embodiment, the interrogation system includes metal interrogators capable of discerning the presence of a metal object, especially in a presence of another metal object.

This application claims the benefit of U.S. Provisional Application No.60/836,997, entitled “Interrogation Systems,” filed on Aug. 11, 2006,and is a continuation in part of U.S. patent application Ser. No.11/357,225 (also U.S. Patent Application Publication No. 2006/0202827),entitled “Interrogator and Interrogation System Employing the Same,”filed Feb. 17, 2006, which is a continuation of U.S. patent applicationSer. No. 10/378,043 (also U.S. Pat. No. 7,019,650, the '650 patent),entitled “Interrogator and Interrogation System Employing the Same,”filed Mar. 3, 2003, and is a continuation in part of U.S. patentapplication Ser. No. 11/071,652 (also U.S. Patent ApplicationPublication No. 2005/0201450, the '450 Publication), entitled“Interrogator and Interrogation System Employing the Same,” filed Mar.3, 2005. All of the aforementioned applications are incorporated byreference.

TECHNICAL FIELD

The present invention is directed, in general, to communication systemsand, more specifically, to an interrogator, method of discerning thepresence of an object, and an interrogation system employing the same.

BACKGROUND

Asset tracking for the purposes of inventory control or the like isemployed in a multitude of industry sectors such as in the foodindustry, apparel markets and any number of manufacturing sectors, toname a few. In many instances, a bar coded tag or radio frequencyidentification (“RFID”) tag is affixed to the asset and a readerinterrogates the item to read the tag and ultimately to account for theasset being tracked. Although not readily adopted, an analogous systemmay be employed in a medical environment to track equipment such as anElectrocardiogram (“EKG”) machine or other modular patient monitoringequipment.

Of particular note is a surgical environment in which for preparationfor surgery a previously sterilized instrument kit of surgicalinstruments and disposable items (collectively referred to as surgicalitems) is brought into a surgical suite. The instrument kit contains anassortment of surgical items including hemostats, clamps, forceps,scissors, sponges, and the like, based on the type of surgery to beperformed. Typically, a scrub nurse removes the surgical items from thekit and arranges them on a back table located behind the operatingtable. The surgical items are organized in rows on rolled toweling forease of access and handling by a surgeon and supporting team. During thecourse of a surgical procedure, the surgical items are often positionedon a “Mayo” stand proximate the operating table, while the unusedsurgical items remain on the back table. During the course of and at theconclusion of the surgery, all of the surgical items must be carefullycounted to, among other things, avoid leaving any surgical items in apatient.

In view of the consequences, surgical items are typically counted atleast three times during the course of a surgical procedure. The firstcount is performed prior to the start of the procedure; the second countis performed prior to a closure of the patient; the third count isperformed at the conclusion of the procedure. In many instances, such aswhen more than one surgical team is assigned to a procedure, many morecounts of the surgical items, often involving different personnel (e.g.,a circulating nurse and a scrub nurse), are performed. As a matter offact, the Association of PeriOperative Registered Nurses (“AORN”)advocates four counts of the surgical items as part of its recommendedpractices for surgical procedures. Additionally, to keep track of thecounts of the surgical items, rudimentary systems such as visual recordsscribbled on whiteboards or other more progressive computer tallyingsystems to designate the count of the surgical items are often employed.

In common practice, access to and from an operating room in the surgicalsuite is restricted during the counting process, thereby resulting in adetention of valuable professional personnel. A discrepancy in the countmust be resolved by additional counts, physical examination of thepatient or x-ray examination, if necessary. Although it is unusual for adiscrepancy in the count to result from a surgical item remaining in thepatient, counting and recounting occurs in every surgical procedure andthe repercussions associated with the loss of a surgical item is ofgrave concern to a medical facility and the professionals.

Thus, the multiple manual counting of surgical items is time consuming,ties up key professional personnel, contributes to surgical suite downtime, distracts personnel from the surgical procedure, lengthens thetime the patient is exposed to anesthesia leading to an increase inmortality and morbidity risk, is generally distasteful to all involved,and still results in errors wherein materials are left in the patient.It should be quite understandable that the average cost overruns of suchdelays associated with the personnel, capital equipment and the surgicalsuite itself can run into the tens of thousands of dollars perprocedure. On an annual basis, the loss of productivity associated withthe surgical suite is quite sizeable and should be addressed to bolsterthe bottom line of a medical facility.

Even with the degree of caution cited above, the problem associated withthe loss of surgical items, especially surgical items retained withinpatients, is a serious one and has a significant influence on the costsof malpractice insurance. As a matter of fact, retained foreign bodieswithin a patient is one of the most prevalent categories of malpracticeclaims and the most common retained foreign body is a sponge. Inaccordance therewith, there is a diagnosis known as “gossypiboma”(wherein gossypium is Latin for cotton and boma is Swahili for place ofconcealment) for the retention of a sponge-like foreign body in apatient. The medical literature is scattered with reports ofpresentations of retained sponges found days, months, or even yearsafter a surgical procedure.

The sponge is typically made of gauze-like material with dimensionsoften covering a four-inch square or a two-inch by four-inch rectangle.At one time sponges were commonly made of cotton, but now a number offilament materials are used. Occasionally, a filament of radiopaquematerial [e.g., barium sulfate (“BaSO₄”)] is woven into the surgicalsponge, or a tab of that material is attached to the surgical sponge.The filament or tab is provided to produce a distinct signature on anx-ray machine for the purpose of determining if a sponge is present inthe patient. While this is generally effective, even these filaments ortabs are not 100% effective in aiding the location of the sponges.Different researchers report that x-ray methods to supplement manualcounting are fallible.

Moreover, in cases when a sponge remains in the body for a long time,the radiopaque filament can become difficult to locate and may evenconform to internal structures. Some have suggested that a computerizedtomography (“CT”) scan can be more effective than an x-ray examinationbecause the CT scans and ultrasonography may detect the reduced densityof a sponge and its characteristic pattern of air or gas bubbles trappedwithin the sponge. Many radiologists have published a number of papersover the years on the problem of finding lost sponges and these aregenerally known in the field of medicine.

As mentioned above, there is a widespread practice in other fields forcounting, tracking and accounting for items and two of the moreprevalent and lowest cost approaches involve various types of bar codingand RFID techniques. As with bar coding, the RFID techniques areprimarily used for automatic data capture and, to date, the technologiesare generally not compatible with the counting of surgical items. Areason for the incompatibility in the medical environment for the barcoding and RFID techniques is a prerequisite to identify items coveredin fluids or waste, and the exigencies associated with the sterilizingof surgical items including a readable tag. Even in view of theforegoing limitations for the application of RFID techniques in themedical environment wherein less than ideal conditions are prevalent,RFID tags have been compatible with a number of arduous environments. Inthe pharmaceutical industry, for instance, RFID tags have survivedmanufacturing processes that require products to be sterilized for aperiod of time over 120 degrees Celsius. Products are autoclaved whilemounted on steel racks tagged with an RFID tag such that a rackidentification (“ID”) number and time/date stamp can be automaticallycollected at the beginning and end of the process as the rack travelsthrough the autoclave on a conveyor. The RFID tags can be specified towithstand more than 1000 hours at temperatures above 120 degreesCelsius. This is just one example of how RFID tags can withstand thearduous environment including the high temperatures associated with anautoclave procedure, whereas a bar code label is unlikely to survivesuch treatment.

While identification tags or labels may be able to survive the difficultconditions associated with medical applications, there is yet anotherchallenge directed to attaching an identification element to a surgicalitem or any small device. The RFID tags are frequently attached todevices by employing mechanical techniques or may be affixed with sewingtechniques. A more common form of attachment of an RFID tag to a deviceis by bonding techniques including encapsulation or adhesion.

While medical device manufacturers have multiple options for bonding,critical disparities between materials may exist in areas such asbiocompatibility, bond strength, curing characteristics, flexibility andgap-filling capabilities. A number of bonding materials are used in theassembly and fabrication of both disposable and reusable medicaldevices, many of which are certified to United States PharmacopoeiaClass VI requirements. These products include epoxies, silicones,ultraviolet curables, cyanoacrylates, and special acrylic polymerformulations.

In many instances, the toughness and versatile properties ofbiocompatible epoxies make them an attractive alternative. Epoxies formstrong and durable bonds, fill gaps effectively and adhere well to mosttypes of substrates. Common uses for medical epoxies include a number ofapplications which require sterilization compatibility such as bondinglenses in endoscopes, attaching plastic tips to tubing in disposablecatheters, coating implantable prosthetic devices, bonding balloons tocatheters for balloon angioplasty, and bonding diamond scalpel bladesfor coronary bypass surgery, to name a few. A wide range of suchmaterials are available and some provide high strength bonds which aretough, water resistant, low in outgassing, and dimensionally stable overa temperature range of up to 600 degrees Fahrenheit. Some epoxies canwithstand repeated sterilization such as autoclaving, radiation,ethylene oxide and cold (e.g., chemical) sterilization methods.

As previously mentioned, familiar applications for RFID techniquesinclude “smart labels” in airline baggage tracking and in many storesfor inventory control and for theft deterrence. In some cases, the smartlabels may combine both RFID and bar coding techniques. The tags mayinclude batteries and typically only function as read only devices or asread/write devices. Less familiar applications for RFID techniquesinclude the inclusion of RFID tags in automobile key fobs as anti-theftdevices, identification badges for employees, and RFID tags incorporatedinto a wrist band as an accurate and secure method of identifying andtracking prison inmates and patrons at entertainment and recreationfacilities. Within the medical field, RFID tags have been proposed fortracking patients and patient files, employee identification badges,identification of blood bags, and process management within thefactories of manufacturers making products for medical practice.

Typically, RFID tags without batteries (i.e., passive devices) aresmaller, lighter and less expensive than those that are active devices.The passive RFID tags are typically maintenance free and can last forlong periods of time. The passive RFID tags are relatively inexpensive,generally as small as an inch in length, and about an eighth of an inchin diameter when encapsulated in hermetic glass cylinders. Recentdevelopments indicate that they will soon be even smaller. The RFID tagscan be encoded with 64 or more bits of data that represent a largenumber of unique ID numbers (e.g., about 18,446,744,073,709,551,616unique ID numbers). Obviously, this number of encoded data provides morethan enough unique codes to identify every item used in a surgicalprocedure or in other environments that may benefit from asset tracking.

An important attribute of RFID interrogation systems is that a number ofRFID tags should be interrogated simultaneously stemming from the signalprocessing associated with the techniques of impressing theidentification information on the carrier signal. A related anddesirable attribute is that there is not typically a minimum separationrequired between the RFID tags. Using an anti-collision algorithm,multiple RFID tags may be readily identifiable and, even at an extremereading range, only minimal separation (e.g., five centimeters or less)to prevent mutual de-tuning is generally necessary. Most otheridentification systems, such as systems employing bar codes, usuallyimpose that each device be interrogated separately. The ability tointerrogate a plurality of closely spaced RFID tags simultaneously isdesirable for applications requiring rapid interrogation of a largenumber of items.

In general, the sector of radio frequency identification is one of thefastest growing areas within the field of automatic identification anddata collection. A reason for the proliferation of RFID systems is thatRFID tags may be affixed to a variety of diverse objects (also referredto as “RFID objects”) and a presence of the RFID tags may be detectedwithout actually physically viewing or contacting the RFID tag. As aresult, multiple applications have been developed for the RFID systemsand more are being developed every day.

The parameters for the applications of the RFID systems vary widely, butcan generally be divided into three significant categories. First, anability to read the RFID tags rapidly. Another category revolves aroundan ability to read a significant number of the RFID tags simultaneously(or nearly simultaneously). A third category stems from an ability toread the RFID tags reliably at increased ranges or under conditionswherein the radio frequency signals have been substantially attenuated.While significant progress has been made in the area of reading multipleRFID tags almost simultaneously (see, for instance, U.S. Pat. No.6,265,962 entitled “Method for Resolving Signal Collisions BetweenMultiple RFID Transponders in a Field,” to Black, et al., issued Jul.24, 2001, which is incorporated herein by reference), there is stillroom for significant improvement in the area of reading the RFID tagsreliably at increased ranges or under conditions when the radiofrequency signals have been substantially attenuated.

Accordingly, what is needed in the art is an interrogator, interrogationsystem and related method to identify and account for all types of itemsregardless of the environment or application that overcomes thedeficiencies of the prior art.

SUMMARY OF THE INVENTION

These and other problems are generally solved or circumvented, andtechnical advantages are generally achieved, by advantageous embodimentsof the present invention which includes an interrogator andinterrogation systems employing the same. In one embodiment, theinterrogation systems include multiple interrogators that communicatewith a base command unit to track a location of an object. In anotherembodiment wherein the object is an RFID object (e.g., an object with anRFID tag), the interrogators employ signal processing techniques such asprecharging the RFID object, and correlating a reference code with areply code from the RIFD object using selected techniques to increase asensitivity of the interrogator, especially for adverse environments. Inother embodiments, the interrogation systems include variations of metalinstruments and sponges employed therewith. In yet another embodiment,the interrogation system includes metal interrogators capable ofdiscerning the presence of a metal object, especially in a presence ofanother metal object.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter which form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand specific embodiment disclosed may be readily utilized as a basis formodifying or designing other structures or processes for carrying outthe same purposes of the present invention. It should also be realizedby those skilled in the art that such equivalent constructions do notdepart from the spirit and scope of the invention as set forth in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, reference isnow made to the following descriptions taken in conjunction with theaccompanying drawings, in which:

FIGS. 1 to 14 illustrate system level diagrams of embodiments ofinterrogation systems constructed according to the principles of thepresent invention;

FIG. 15 illustrates a functional block diagram of an embodiment of aninterrogation system constructed according to the principles of thepresent invention;

FIG. 16 illustrates a block diagram of an embodiment of a base commandunit constructed according to the principles of the present invention;

FIG. 17 illustrates a block diagram of an embodiment of an interrogatorconstructed according to the principles of the present invention;

FIGS. 18 to 28 illustrate diagrams of exemplary antennas employable withan interrogation system constructed according to the principles of thepresent invention;

FIG. 29 illustrates a block diagram of another embodiment of aninterrogation system demonstrating the capabilities associated withradio frequency identification according to the principles of thepresent invention;

FIG. 30 illustrates a block diagram of another embodiment of aninterrogator constructed in accordance with the principles of thepresent invention;

FIG. 31 illustrates a block diagram of an embodiment of portions of theRFID sensing subsystem of FIG. 30 constructed in accordance with theprinciples of the present invention;

FIG. 32 illustrates a waveform diagram of an exemplary response from anRFID tag of an RFID object in accordance with the principles of thepresent invention;

FIG. 33 illustrates a waveform diagram of a spectral response associatedwith the response from the RFID tag illustrated in FIG. 32;

FIG. 34 illustrates a block diagram of portions of a control andprocessing subsystem of an interrogator constructed according to theprinciples of the present invention;

FIG. 35 illustrates a block diagram of an embodiment of portions of acorrelation subsystem associated with a control and processing subsystemof an interrogator demonstrating an exemplary operation thereof inaccordance with the principles of the present invention;

FIG. 36 illustrates a waveform diagram demonstrating exemplaryadvantages associated with the correlation subsystem described withrespect to FIGS. 34 and 35;

FIG. 37 illustrates a waveform diagram demonstrating the sidelobesassociated with the correlation subsystem in accordance with theprinciples of the present invention;

FIG. 38 illustrates a block diagram of portions of an embodiment of acontrol and processing subsystem of an interrogator constructedaccording to the principles of the present invention;

FIG. 39 illustrates a waveform diagram demonstrating an application of acell averaging constant false alarm rate with an interrogator accordingto the principles of the present invention;

FIG. 40 illustrates a waveform diagram of a cell under test to a cellaverage ratio as a function of a cell under test lag using an unfilteredreference in accordance with a constant false alarm rate in accordancewith the principles of the present invention;

FIG. 41 illustrates a waveform diagram of a cell under test to a cellaverage ratio for another constant false alarm rate in accordance withthe principles of the present invention;

FIGS. 42 and 43 illustrate diagrams of embodiments of filters employablewith a control and processing subsystem of an interrogator constructedaccording to the principles of the present invention;

FIG. 44 illustrates a waveform diagram of a response from a low passfilter employable with a control and processing subsystem of aninterrogator constructed according to the principles of the presentinvention;

FIG. 45 illustrates a diagram of a filter structure employable with acontrol and processing subsystem of an interrogator constructedaccording to the principles of the present invention;

FIGS. 46 and 47 illustrate diagrams of an interrogation sequence inaccordance with an interrogator;

FIG. 48 illustrates an embodiment of an interrogation sequence inaccordance with an interrogator constructed according to the principlesof the present invention;

FIG. 49 illustrates waveform diagrams of an embodiment of aninterrogation sequence from an interrogator, along with two RFID tagresponse waveforms from RFID tags designated Tag A and Tag B inaccordance with the principles of the present invention;

FIG. 50 illustrates waveform diagrams of an embodiment of aninterrogation sequence from an interrogator, response for an RFID tag,and correlation signals from a correlation subsystem of an interrogatorin accordance with the principles of the present invention;

FIG. 51 illustrates waveform diagrams of an embodiment of full RFID tagresponses and a partial RFID tag response in accordance with theprinciples of the present invention;

FIG. 52 illustrates waveform diagrams of an embodiment for a string ofmodulated binary zeros and for a string of modulated binary ones asencoded using frequency shift keying in accordance with the principlesof the present invention;

FIG. 53 illustrates waveform diagrams of an embodiment of referencecodes or RFID tag signatures including data and clock signals orinformation, clock-only signals or information, and data-only signals orinformation in accordance with the principles of the present invention;

FIG. 54 illustrates a waveform diagram of an embodiment of anon-coherently integrated correlation response for an RFID tag matchinga reference code or signature in accordance with an interrogatorconstructed according to the principles of the present invention;

FIG. 55 illustrates waveform diagrams of an embodiment of anon-coherently integrated correlation response for an RFID tag thatmatches a reference code or signature, for an RFID tag with a similarreply code to the reference code, and for no RFID tag present inaccordance with an interrogator constructed according to the principlesof the present invention;

FIGS. 56 and 57 illustrate flow diagrams of embodiments of methods ofoperating an interrogation system according to the principles of thepresent invention;

FIGS. 58 and 59 illustrate diagrams of embodiments of an RFID tagaccording to the principles of the present invention;

FIG. 60 illustrates pictorial representations of metal instruments(e.g., medical instruments) employable with the interrogation system ofthe present invention;

FIGS. 61 and 62 illustrate top and side views of an embodiment of ametal instrument including an RFID tag according to the principles ofthe present invention;

FIG. 63 illustrates a side view of an embodiment of a metal instrumentincluding an RFID tag according to the principles of the presentinvention;

FIG. 64 illustrates a pictorial representation of an exemplary countingsystem for surgical sponges;

FIGS. 65 to 68 illustrate pictorial representations of several types ofsurgical sponges;

FIG. 69 illustrates pictorial representations of RFID tags;

FIGS. 70 to 77 illustrate diagrams of embodiments of a sponge inaccordance with the principles of the present invention;

FIGS. 78 and 79 illustrate block diagrams of exemplary environments forapplication of a metal interrogator in accordance with the principles ofthe present invention;

FIGS. 80 and 81 illustrate block diagrams of an embodiment of a metalinterrogator constructed according to the principles of the presentinvention;

FIGS. 82 to 89 illustrate diagrams of embodiments of antenna arraysemployable with a metal interrogator constructed according to theprinciples of the present invention;

FIG. 90 illustrates a functional block diagram of portions of anembodiment of a metal interrogator constructed according to theprinciples of the present invention;

FIG. 91 illustrates digitized waveform diagrams demonstrating waveformsproduced by a metal interrogator in accordance with the principles ofthe present invention;

FIG. 92 illustrates exemplary waveform diagrams of three sampledresponses taken from the same antenna within a short time interval inaccordance with the principles of the present invention;

FIG. 93 illustrates waveform diagrams of intra-sample noise reductionthat relies upon the fact that the resultant waveform from an inductionpulse is an exponential curve in accordance with the principles of thepresent invention;

FIGS. 94 and 95 illustrate flow diagrams of embodiments of a residualmethod metal detection process for a metal interrogator in accordancewith the principles of the present invention;

FIG. 96 illustrates waveform diagrams of sampled responses from twoantennas, which are digitized and sampled, according to the principlesof the present invention; and

FIG. 97 illustrates waveform diagrams for sampled responses from twodifferent metal detection antennas in accordance with the principles ofthe present invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the presently preferred embodiments arediscussed in detail below. It should be appreciated, however, that thepresent invention provides many applicable inventive concepts that canbe embodied in a wide variety of specific contexts. The specificembodiments discussed are merely illustrative of specific ways to makeand use the invention, and do not limit the scope of the invention. Thepresent invention will be described with respect to exemplaryembodiments in a specific context, namely, an interrogator, methods ofdiscerning metal objects (e.g., objects that include metal), RFIDobjects (e.g., objects that include an RFID tag or radio frequencyidentification), and other objects, and an interrogation systememploying the same. The principles of the present invention areapplicable to may fields including, without limitation, the medicalenvironment, supply chain management systems in the retail industry, andthe defense industry.

Referring initially to FIGS. 1 to 14, illustrated are system leveldiagrams of embodiments of interrogation systems constructed accordingto the principles of the present invention. The interrogation systemsinclude multiple interrogators (designated “INT”) with correspondingantennas (designated “ANT”) that define an active area for detecting,without limitation, RFID objects (e.g., objects such as a sponge with anRFID tag attached thereto), metal objects (e.g., objects includingmetal), and bar coded objects (e.g., objects such as a blood bag with abar code thereon). The interrogators illustrated with respect to theinterrogation systems of FIGS. 1 to 7 include far field antennas. Theinterrogators are located at stations (such as a back table, a soiledconsumable (or disposable) and instrument station, a dirty basinstation, and an operating station of an operating room).

Additionally, ones of the interrogators form mobile interrogators with,for instance, an RFID wand (designated “RFID WAND;” see, e.g., FIG. 2),a metal wand (designated “MTL WAND;” see, e.g., FIG. 3), an integratedRFID and metal wand (designated “RFID/MTL WAND;” see, e.g., FIG. 5) orcombinations thereof, located at a station such as an operating station.As illustrated in FIG. 6, selected ones of the mobile interrogators maybe integrated RFID and metal interrogator wands (designated “RFID/MTLINT WAND”) chargeable through a charger (designated “CHARGER”). Whilethe illustrated mobile interrogators include RFID and metal wands, itshould be understood that other technologies may be employed inconjunction with the mobile interrogators such as, without limitation,bar code, optical, optical recognition, microelectromechanical systems,radio frequency and dot-peening. As an example, optical scanning may beemployed to detect and account for small items such as needles.

The interrogators are coupled to a base command unit (e.g., wirelesslyas illustrated in FIG. 4) such as personal computer, a laptop computer,a server, or any computer processing system. The base command unit(designated “BCU”) is coupled to another computer system (designated“CS”) such as a hospital information technology system and a display(e.g., a wall mounted display designated “DIS”). The base command unitincludes control and processing subsystems for the interrogation system.

As illustrated in FIG. 7, the interrogation system may also incorporatemultiple interrogators (designated “INT1” and “INT2”) employingdifferent types of technologies that accommodate different technologiessuch as, without limitation, RFID, metal, bar code, optical, opticalrecognition, microelectromechanical systems, radio frequency anddot-peening. Of course, the function of detecting different types oftechnologies may be integrated into a single interrogator. Thus, theinterrogation systems can detect, count and account for objects such as,without limitation, RFID objects (e.g., object with an RFID tag), metalobjects and bar coded objects (e.g., objects with a bar code). Theinterrogation systems allow an RFID object or other object to be readinto the base command unit via an interrogator and reconciled, at alater time, by being read by the same or a different interrogator intothe base command unit. The base command unit receives signals from theinterrogators to track a location of an object, preferably in real time,which may be shown on a display. As illustrated in FIG. 8, theinterrogators are operable with different types of antennas such as,without limitation, near field antennas (designated “ANT_(NF)”), ringantennas (designated “ANT_(RING)”), near field antenna arrays(designated “ANT_(NF ARRAY)”) and bi-static antennas.

Another problem encountered is the ability to detect and read an RFIDobject at any location within a defined area. One method ofaccomplishing this function is to specifically place an antenna, forexample, over a specific area so that that area may be monitored.However, in doing so, other RFID objects that may be in areas adjacentto the desired area can also be inadvertently read due to coverageoverlap in the antennas thereby generating erroneous data. In addition,some environments strictly forbid antennas over areas wherein objectsneed to be detected. Also, the dimensions of the specific areas are notstandardized and are required in various sizes and shapes that are notdefined apriori. Therefore, what is needed is a interrogation systemcapable of reliably detecting and counting RFID objects in a welldefined area while not erroneously counting them in adjacent areas andadditionally able to easily define and modify this area both in size andshape.

Turning now to FIG. 9, illustrated is a diagram of an interrogationsystem capable of providing interrogation (such as RFID interrogation)coverage to a specific and well defined area. An array of tiles (a tileof which is designated “TILE”) is located on a surface of a table(designated “TABLE”). Each tile is individually capable of detecting andcounting an object (e.g., an RFID object) if the RFID object is placedon or above that tile. The array is connected to an interrogator(designated “INT”) via cabling that individually directs each tile tointerrogate its specific area and reports the results back to a basecommand unit via cabling. The individual tiles are reconfigurable withrespect to each other so that both the shape and size of the active areacan be easily changed.

Turning now to FIG. 10, illustrated is a diagram of an embodiment of anactive tile according to the principles of this invention. The activetile (designated “Active Tile”) includes an integrated interrogator(designated “INT”) connected to an antenna (designated “ANT”). Theinterrogator derives prime power (designated “Pwr”) and input commandsas well as outputting results (via an input/output bus designated “I/O”)digitally to a controller.

Turning now to FIG. 11, illustrated is a diagram of an embodiment of anactive tile (designated “Active Tile”) illustrating how an interrogator(designated “INT”) is connected directly to an antenna (designated“ANT”). The side of the antenna opposite that side to which theinterrogator is connected forms the radiating face of the antenna.Multiple tiles can be mounted side by side in various configurations toestablish a desired area and size of desired coverage.

Turning now to FIG. 12, illustrated is a diagram wherein active tiles(designated “Active Tile”) are individually addressable via a bus(designated “BUS”). The active tiles communication via the bus with acontroller (designated “CTLR”) that includes a digital multiplexer(designated “DMUX”) connected to a translator (designated “TNLR”), whichcommunicates with a base command unit. The controller polls the activetiles to obtain the results therefrom and provides a translation forcommunication with the base command unit.

Turning now to FIG. 13, illustrated is a diagram wherein active tiles(designated “Active Tile”) are individually addressable via separatebuses (one of which is designated “BUS”). The active tiles communicatevia their respective bus with a controller (designated “CTLR”) thatincludes a digital multiplexer (designated “DMUX”) connected to atranslator (designated “TNLR”), which communicates with a base commandunit. The controller polls the active tiles to obtain the resultstherefrom and provides a translation for communication with the basecommand unit.

Turning now to FIG. 14, illustrated is a diagram wherein an antenna oftiles (designated “TILE”) individually communicate via an RF link (oneof which is designated “RFL”) to an RF multiplexer (designated “RMUX”)of a controller (designated “CTLR”). The controller also includes aninterrogator (designated “INT”) and a translator (designated “TNLR”),which communicates with a base command unit. Thus, the controllertransmits/receives RF signals from the tiles, performs the interrogationfunction, and provides a translation for communication with the basecommand unit.

In addition to logging the continuous presence of an object such as anRFID object within a given area as discussed above, it is also oftenrequired to log items in at individual stations. Again, it is importantto reliably perform the logging function for the desired objects whilenot erroneously detecting and therefore erroneously logging in anyundesired objects. Therefore, what is needed is an interrogation systemthat reliably and easily logs in desired objects while automaticallyrejecting extraneous and, therefore, undesired objects that may be inthe vicinity.

Turning now to FIG. 15, illustrated is a functional block diagram of anembodiment of an interrogation system constructed according to theprinciples of the present invention. The interrogation system includes abase command unit (designated “BCU”) including an input/output module(designated “I/O module” including, without limitation, a display ormonitor, keyboard, mouse and printer), a device management module(designated “DM module” including, without limitation, a user interface,accountable object management, remote scanner/detection management,fault tolerance, external interfaces, continuous diagnostics,maintenance and configuration), an operating system (designated “O/Smodule” such as an MS Windows operating system), a processor (designated“processor”), and memory (designated “memory” including, withoutlimitation, a hard drive). The base command unit also includes anexternal computer system interface (designated “CS I/F” including,without limitation, a PCMCIA interface) to external computer systemssuch as, without limitation, an information technology system or supplychain management system. The base command unit also includes a powermanagement module (designated “PM module”) coupleable to an externalsource of power. The base command unit also includes a wand charger(designated “wand charger”) coupleable to a wand when docked. The basecommand unit also includes a remote communications module (designated“RC module”) to communicate with the wand and scan units via, forinstance, bluetooth. In the illustrated embodiment, the wand unitrepresents a mobile interrogator and the scan unit represents a fixedinterrogator.

The wand unit (designated “wand”) and the scan unit (designated“scanner”) include a device management module (designated “DM module”)including, without limitation, device state management andconfiguration, maintenance interface, fault tolerance, externalinterfaces and continuous diagnostics. The wand and scan units alsoinclude an RFID sensing subsystem (designated “RFID S/S”) coupled to anRFID antenna (designated “RFID ANT”) via a radio frequency poweramplifier (designated “RF PA”). The wand and scan units also include ametal sensing subsystem (“metal S/S”) coupled to a metal antenna(designated “metal ANT”) and another sensing subsystem (“other S/S”including, without limitation, bar code, optical, optical recognition,microelectromechanical systems, radio frequency and dot-peening) coupledto another antenna/interface (designated “other ANT/IF”). In theillustrated embodiment, the wand and scan units are detecting orscanning for an RFID object (designated “RFID object” such as, withoutlimitation, a sponge with an RFID tag) via radio frequency (“RF”)energy. Of course, an analogous principles apply for metal objects orother objects as well.

The wand and scan units also include a user interface module (designated“UI module”) and a remote communications module (designated “RC module”)to communicate with the base command unit. The scan unit also includes apower management module (designated “PM module”) coupleable to anexternal source of power and the wand unit includes a power managementmodule (designated “PM module”) coupleable to a charger.

Turning now to FIG. 16, illustrated is a block diagram of an embodimentof a base command unit constructed according to the principles of thepresent invention. The base command unit includes a processor, a videoprocessor, memory (e.g., hard drive and memory backup), a router (e.g.,a 12 port universal serial bus 2.0 router), a power supply, andinput/output devices such as a monitor or display, a printer, an audibledevice, a keyboard and a pointing device.

Turning now to FIG. 17, illustrated is a block diagram of an embodimentof an interrogator constructed according to the principles of thepresent invention. The interrogator includes a metal sensing subsystem(including a metal detection wand assembly and metal detector), an RFIDsensing subsystem (including an RFID reader module, fixed RFID antennaor wand assembly, power amplifier, digital and analog assemblies), ahost communication and power management module, a power supply andremote communications module such as a wireless interface. While theillustrated interrogator includes RFID and metal sensing subsystems andmodules, it should be understood that the interrogator may include othersensing subsystems and modules such as, without limitation, bar code,optical, optical recognition, microelectromechanical systems, radiofrequency and dot-peening.

Turning now to FIGS. 18 to 28, illustrated are diagrams of exemplaryantennas employable with an interrogation system constructed accordingto the principles of the present invention. Often in detection, anobject (e.g., an RFID object) to be detected is fixed and theinterrogator is scanned over an area where an RFID tag of the RFIDobject might be located. One approach to this problem is to simply usean existing antenna in a monostatic configuration and extend it viaradio frequency (“RF”) cabling so that the antenna can in effect bescanned over an area. The problem with this approach is that should theantenna be of a near field design, its range may only be on the order ofa few inches and therefore render it ineffective for many applications.On the other hand, a conventional bistatic antenna using far fieldantennas is large and the area of detection can also be sufficientlylarge so as to render it useless for any detection that also requireslocation information. Therefore, what is needed is an antenna that bothprovides good location information and maintains good detectionsensitivity beyond several inches.

Turning now to FIG. 18, illustrated is a diagram of an RFID interrogator(designated “INT”) employing a bistatic antenna configuration where afirst antenna (designated “RX ANT”) is connected to a receiver port(designated “RX”) and a second antenna (designated “TX ANT”) isconnected to a transmitter port (designated “TX”). This approach allowsfor greater isolation between the transmitted and received signals andalso allows for different antenna characteristics for the transmit andreceive functions of the interrogator.

Turning now to FIG. 19, illustrated is an embodiment of a bistaticantenna configuration used with RFID interrogator specificallyconfigured to provide both good detection sensitivity at ranges ofseveral feet while also maintaining good location detection capability.In this embodiment, a transmit antenna (designated “TX ANT”) isconnected to the transmit port (designated “TX”) of the interrogator andis a single antenna about which is placed two (2) receive antennas(designated “RX ANT”) which are then connected to a matching network(designated “MN”) and then to the receive port (designated “RX”) of theinterrogator. The transmit antenna and receive antennas need not be ofthe same type as illustrated in this embodiment. By mounting thebistatic antennas in this configuration, in addition to improved RFisolation between transmit and receive ports, the interrogator attainssubstantially greater capability for specifically locating the RFIDobject with respect to a known reference point (designated “REF”) thatis uniquely defined.

Turning now to FIGS. 20 to 26, illustrated are embodiments of antennaconfigurations constructed according to the principles of the presentinvention. Regarding FIGS. 20 and 21, a curved metallic structurecontains a single continuous element (designated “CE”). This structure,therefore, places a much higher field strength for detection within thecurved structure than without. Additionally, a mounting structure(designated “MT”) such as a pole supports the antenna. Of course, othermounting structures are well within the broad scope of the presentinvention. As illustrated in FIG. 22, the curved metallic structure hasbeen replaced by straight surfaces. This antenna configuration offers amore open configuration so that the object can be logged at greaterdistances. As illustrated in FIGS. 23 and 24, the continuous elementdiscussed above is replaced by individual discrete elements (designated“DE”) and mounted to the curved conducting surfaces. As illustrated inFIG. 25, individual discrete elements (designated “DE”) are located onstraight conducting surfaces.

Turning now to FIG. 26, a closed metal structure having an open top(designated “OT”) and open bottom has mounted onto its inner surfacesdiscrete antenna elements (designated “DE”). In this manner, an objectwhen passing through the structure shall have a higher probability ofbeing detected due the multiple opportunities for the RFID object to beboth illuminated and read by the multiple antennas and the RF multipathgenerated within this relatively closed environment. This environmentalso provides excellent localized RFID interrogation, while at the sametime not extraneously reading RFID objects in adjacent areas.

Turning now to FIGS. 27 and 28, illustrated are other embodiments ofantenna configurations constructed according to the principles of thepresent invention. In addition to detecting an RFID object when passingtherethrough, the antenna configurations may also detect a direction bywhich that RFID object passed therethrough. Applications of this addedcapability include the ability to accept or log in an object by placingthe object therethrough in one direction and rejecting or logging out anobject by placing the object therethrough in the opposite direction.Other applications for this added capability are comprehended herein.

Regarding FIG. 27, the antenna configuration is a generally cylindricalmetallic structure open at both ends (one of which is designated “END”)and includes two bands of antenna elements (designated “AEL”), which maybe either continuous or discrete. Additionally, an RF barrier(designated “RFB”) is also included so that radiation from one antennaelement (or set thereof) does not impinge or excite the other antennaelement (or set thereof). Therefore, an object passing through thestructure is detected first by one antenna element and then the other atdifferent times, defining uniquely the direction of the RFID object.

Regarding FIG. 28, the cylindrical shape discussed above is replaced bya generally square shape open at both ends (one of which is designated“END”). Here, the antenna elements (designated “AEL”) also encompass thestructure and are also separated by an RF barrier (designated “RFB”). Itshould be noted that the exemplary embodiments of the antennasintroduced herein are provided for illustrative purposes only and otherembodiments that include arrays for both transmit and receive antennasas well as antennas mounted in different configurations with respect toeach other to achieve specific desirable properties such as thatdiscussed above for specific applications are well within the broadscope of the present invention.

Thus, an interrogation system and method of operating the same has beenintroduced herein. In an aspect, the interrogation system includes aninterrogator configured to detect an object at a first time and a secondtime, and a base command unit is configured to receive signals from theinterrogator to track a location of the object, advantageously in realtime. In another aspect, the interrogation system includes a first fixedinterrogator, located at a first station, configured to detect an objectat a first time, a second fixed interrogator, located at a secondstation, configured to detect the object at a second time, and a mobileinterrogator, located at a third station, configured to detect theobject at a third time. A base command unit of the interrogation systemis configured to receive signals from the first and second fixedinterrogators and the mobile interrogator to track a location of theobject, advantageously in real time. The base command unit may becoupled to a display to show a location of the object and communicatewith another computer system.

In a related embodiment, the object is selected from the groupconsisting of a radio frequency identification object, a radio frequencyobject, a metal object, a bar coded object, a microelectromechanicalsystems object, an optical recognition object and a dot-peening object.Additionally, the interrogators employ antennas selected from the groupconsisting of far field antennas, near field antennas, near fieldantenna arrays, ring antennas and bi-static antennas. In a medicalenvironment, ones of the locations mentioned above are located in anoperating room selected from the group consisting of a back table, asoiled consumable and instrument station, a dirty basin station and anoperating station. In accordance therewith, the object may be a radiofrequency identification object in the form of a sponge with a radiofrequency identification tag.

In another related embodiment, ones of the interrogators are radiofrequency identification interrogators and the object is a radiofrequency identification object and the interrogation system furtherincludes another interrogator configured to detect a different object.Thus, the base command unit can receive signals from the anotherinterrogator to track a location of the different object. In a relatedembodiment, ones of the interrogators are an integrated radio frequencyidentification and other interrogator configured to detect a radiofrequency identification object and a different object. Thus, the basecommand unit is configured to receive signals from the interrogator totrack a location of the radio frequency identification and differentobjects.

In one aspect, the base command unit is a laptop computer. Additionally,the base command unit may include an input/output device, a devicemanagement module, an operating system, a processor, a memory, anexternal computer system interface, a power management module and aremote communications module. Also, ones of the interrogators mayinclude a device management module, a sensing subsystem, a powermanagement module, a remote communications module and a user interfacemodule. In applications wherein the object is a radio frequencyidentification object, ones of the interrogators generate radiofrequency energy in accordance with a radio frequency identificationsensing subsystem and a radio frequency power amplifier to detect theradio frequency identification object. Additionally, the interrogationsystem may include a charger for the mobile interrogator mentionedabove.

Turning now to FIG. 29, illustrated is a block diagram of anotherembodiment of an interrogation system demonstrating the capabilitiesassociated with radio frequency identification according to theprinciples of the present invention. The interrogation system includesan interrogator 2915 including an RFID sensing subsystem 2920 and acontrol and processing subsystem 2930 that energizes an RFID tag 2905and then receives, detects and decodes the encoded RF energy (reflectedor transmitted) from the RFID tag 2905. The control and processingsubsystem 2930 provides overall control of the functions of theinterrogator 2915 as well as any reporting functions. The interrogator2915 may also include a user interface, communications subsystem, apower source and other subsystems as described above.

Additionally, the interrogation system may be employed with multipleRFID objects and with different types of RFID tags. For example, theRFID tags may be passive, passive with active response, and fullyactive. For a passive RFID tag, the transmitted energy provides a sourceto charge an energy storage device within the RFID tag. The storedenergy is used to power a response from the RFID tag wherein a matchingimpedance and thereby a reflectivity of the RFID tag is altered in acoded fashion of ones (“1”) and zeros (“0”). At times, the RFID tag willalso contain a battery to facilitate a response therefrom. The batterycan simply be used to provide power for the impedancematching/mismatching operation described above, or the RFID tag may evenpossess an active transmitting function and may even respond at afrequency different from a frequency of the interrogator. Any type oftag (e.g., RFID tag) whether presently available or developed in thefuture may be employed in conjunction with the interrogation system.Additionally, the RFID objects may include more than one RFID tag, eachcarrying different information (e.g., object specific or sensorsreporting on the status of the object) about the RFID object. The RFIDtags may also include more than one integrated circuit, each circuitincluding different coded information for a benefit of the interrogationsystem.

Referring to FIG. 30, illustrated is a block diagram of anotherembodiment of an interrogator constructed in accordance with theprinciples of the present invention. The interrogator includes a metalsensing subsystem 3005, a metal sensing antenna interface 3010, a metalsensing antenna 3015, an RFID sensing subsystem 3020, an RFID sensingantenna interface 3030, an RFID sensing antenna 3035 and a control andprocessing subsystem 3040. While the illustrated embodiment provides foran integrated metal and RFID detection capability, those skilled in theart should understand that portions of the interrogator may be omittedor rendered inactive to provide a metal or RFID interrogator.Additionally, different types of sensing subsystems may be incorporatedinto the interrogator to detect other types of objects, as well.

The metal sensing subsystem 3005 includes a metal sensingdigital-to-analog converter (“DAC”) 3006, a metal sensing transmitamplifier 3007, a metal sensing receive amplifier 3008 and a metalsensing analog-to-digital converter (“ADC”) 3009. The metal sensingantenna interface 3010 includes a metal sensing transmit conditioningfilter 3011 and a metal sensing receive conditioning filter 3012. Themetal sensing antenna 3015 includes a metal sensing transmit antenna3016 and a metal sensing receive antenna 3017.

The RFID sensing subsystem 3020 includes an RFID sensing DAC 3021, anRFID sensing transmit selector switch 3022, a first RFID sensingtransmit amplifier 3023, a second RFID sensing transmit amplifier 3024,a first RFID sensing receive amplifier 3025, a second RFID sensingreceive amplifier 3026, an RFID sensing receive selector switch 3027 andan RFID sensing ADC 3028. The RFID sensing antenna interface 3030includes first and second RFID sensing transmit conditioning filters3031, 3032 and first and second RFID sensing receive conditioningfilters 3033, 3034. The RFID sensing antenna 3035 includes first andsecond RFID sensing transmit antennas 3036, 3037 and first and secondRFID sensing receive antennas 3038, 3039. “HI band” and “LO band”capabilities are present to accommodate the wide frequency rangenecessary to detect the various types of RFID tags.

In an alternative embodiment, a mixing or heterodyning function may beincluded within the RFID sensing ADC 3028 or the RFID sensing DAC 3021functions. These techniques are known to those skilled in the pertinentart and may be employed to translate signal processing to a moredesirable frequency range thereby allowing less expensive or morereadily available components to be used. Additionally, the specificnature and function of the first and second transmit conditioningfilters 3031, 3032 and first and second RFID sensing receiveconditioning filters 3033, 3034 may vary depending on the specificalgorithms employed for control and processing and for signal generationand recovery. Also, some embodiments may not require some or all of thefilters shown.

In the illustrated embodiment, the control and processing subsystem 3040may be a software defined structure that allows features and functionsof the interrogator to be easily modified or tailored by alteringsoftware functions. The control and processing subsystem 3040 employs acrystal oscillator to provide a precise frequency reference for both themetal and RFID sensing subsystems 3005, 3020.

The control and processing subsystem 3040 generates a metal sensingdigital excitation signal based on a metal sensing mode of operationselected and provides this signal to the metal sensing DAC 3006. Themetal sensing digital excitation signal may be in the form of acontinuous tone. Alternatively, the digital excitation signal may varyin amplitude, frequency, or phase and may also be of a pulsed naturewherein the waveform duty cycle is less than 100 percent. The frequencyof the metal sensing digital excitation signal may generally be in therange of five to 100 kilohertz (“kHz”). Different waveforms may be usedto optimize a detection of both ferrous and non-ferrous metals. Thesewaveforms may be selected for different sizes and masses of metals andfor metals at different locations and depths within a patient.Algorithmic information employed in generating these excitation signalsmay be part of the control and processing subsystem 3040.

The metal sensing DAC 3006 converts the metal sensing digital excitationsignal into an analog signal that, except for its amplitude, is themetal sensing transmit signal. The analog signal is provided to themetal sensing transmit amplifier 3007, which amplifies the analog signalto a correct amplitude for transmission. The output of the metal sensingtransmit amplifier 3007 is provided to the metal sensing transmitconditioning filter 3011, which sufficiently attenuates all out-of-bandsignals and provides a proper impedance match to the metal sensingtransmit antenna 3016. The metal sensing transmit antenna 3016 launchesthe metal sensing transmit signal.

A metal object present in the vicinity of the metal sensing transmitantenna 3016 and the metal sensing transmit signal will generate a metalsensing return signal wherein the metal sensing return signal may bebased on a change in a field characteristic of the metal sensingtransmit signal. The field characteristic may be altered in the vicinityof the metal object such that a distinctive metal sensing receive signalimpinges on and excites the metal sensing receive antenna 3017. Theoutput of the metal sensing receive antenna 3017 is provided to themetal sensing receive conditioning filter 3012, which sufficientlyattenuates all out-of-band energy and provides a proper impedance matchbetween the metal sensing receive antenna 3017 and the metal sensingreceive amplifier 3008.

The metal sensing receive amplifier 3008 amplifies the metal sensingreceive signal to a level sufficient for processing and provides it tothe metal sensing ADC 3009. The metal sensing ADC 3009 provides a metalsensing digital signal, proportional to the metal sensing receivesignal, to the control and processing subsystem 3040, which determinesif the metal sensing digital signal has a signature representing apresence of a metal object in the vicinity of the metal sensing antenna3015.

The control and processing subsystem 3040 generates an RFID sensingdigital excitation signal based on an RFID mode of operation selectedand outputs this signal to the RFID sensing DAC 3021. The RFID sensingdigital excitation signal may be in the form of a code that excites andenergizes an RFID object present including an RFID tag. The carrierfrequency associated with this code may be in one of two frequencybands. A first frequency band may be centered around 133-135 kHz and isdesignated as the “LO band.” A second frequency band may be centeredaround 10-13 megahertz (“MHz”) and is designated the “HI band.”Alternatively, a “HI band” around 902-928 MHz may also be employed.Alternatively, the 133-135 kHz and the 10-13 MHz bands may be combinedin the “LO band” and some specific implementations may require only asingle band. A frequency band is selected based on the RFID mode ofoperation selected. Each frequency band corresponds to different typesof RFID tags present, which may be based on its size or other factors.Of course, the broad scope of the present invention is not limited to aparticular frequency band. Generally, algorithmic information togenerate the RFID sensing digital excitation signal is contained in thecontrol and processing subsystem 3040.

The RFID sensing DAC 3021 converts the RFID sensing digital excitationsignal into an analog signal that, except for amplitude, is the RFIDsensing transmit signal. The RFID sensing transmit signal is provided tothe RFID sensing transmit selector switch 3022, which is controlled bythe control and processing subsystem 3040. The RFID sensing transmitselector switch 3022 directs the RFID sensing transmit signal to thefirst RFID sensing transmit amplifier 3023 or the second RFID sensingtransmit amplifier 3024, respectively, based on whether the RFID sensingtransmit signal is “HI band” or “LO band.” The first RFID sensingtransmit amplifier 3023 and the second RFID sensing transmit amplifier3024 increase the amplitude of the “HI band” and “LO band” signals to acorrect amplitude for transmission.

The first RFID sensing transmit amplifier 3023 provides the “HI band”signal to the first RFID sensing transmit conditioning filter 3031 andthe second RFID sensing transmit amplifier 3024 provides the “LO band”signal to the second RFID sensing transmit conditioning filter 3032. Thefirst and second RFID sensing transmit conditioning filters 3031, 3032employ differing center frequencies and sufficiently attenuateassociated out-of-band signals. Additionally, they provide a properimpedance match to their respective first or second RFID sensingtransmit antennas 3036, 3037, which launch their respective RFID sensingtransmit signals.

An RFID object, including an RFID tag, in the vicinity of the first orsecond RFID sensing transmit antenna 3036, 3037 generates an RFIDsensing return signal. The RFID sensing return signal impinges on andexcites the appropriate first or second RFID sensing receive antenna3038, 3039, respectively, to provide an RFID sensing receive signal. Anoutput of the first or second RFID sensing receive antenna 3038, 3039 isprovided to the first or second RFID receive conditioning filter 3033,3034, respectively. The first or second RFID receive conditioning filter3033, 3034 sufficiently attenuates the out-of-band energy and provides aproper impedance match between the first or second RFID sensing receiveantenna 3038, 3039 and the first or second RFID sensing receiveamplifier 3025, 3026, respectively.

The first or second RFID sensing receive amplifier 3025, 3026 amplifiesthe small RFID sensing receive signal to a level sufficient forprocessing and provides an amplified RFID sensing receive signal to theRFID sensing receive selector switch 3027, which is controlled by thecontrol and processing subsystem 3040. The control and processingsubsystem 3040 selects the appropriate reception path through the RFIDsensing receive selector switch 3027 for input to the RFID sensing ADC3028, based on the excitation signal transmitted. The RFID sensing ADC3028 provides an RFID sensing digital signal, proportional to the RFIDsensing receive signal, to the control and processing subsystem 3040,which determines if the RFID sensing receive signal has a signaturerepresenting a presence of an RFID object in the vicinity of the RFIDsensing antenna 3035. For an example of such an interrogator, see U.S.Pat. No. 7,019,650 (the '650 patent), entitled “Interrogator andInterrogation System Employing the Same,” to Volpi, et al., issued Mar.28, 2006, which is incorporated herein by reference.

Turning now to FIG. 31, illustrated is a block diagram of an embodimentof portions of the RFID sensing subsystem 3020 of FIG. 30 constructed inaccordance with the principles of the present invention. For purposes ofillustration, the illustrated embodiment and related description isdirected to the “Hi band” signals. Of course, the principles describedherein are equally applicable to the “LO band” signals. Morespecifically, the illustrated embodiment provides an exemplary RFIDsensing transmit amplifier (e.g., the first RFID sensing transmitamplifier 3023) of the RFID sensing subsystem 3020 of FIG. 30 and willhereinafter be described with continuing reference thereto.

The first RFID sensing transmit amplifier 3023 includes first and secondamplifiers 3052, 3062, a mixer 3056 and an oscillator 3058. The firstamplifier 3052 (acting as a buffer and amplifier) receives a signal fromthe RFID sensing transmit selector switch 3022 and provides a modulatedsignal to the mixer 3056. The oscillator 3058 receives a control signalfrom the control and processing subsystem 3040 and provides a carriersignal to the mixer 3056 to set an RF carrier frequency. The controlsignal determines the basic RF carrier frequency that can changeaccording to a specific air interface specification. For example, theUnited States ultra-high frequency (“US UHF”) standard specifies a basiccarrier frequency between 902 and 928 MHz. Of course, other standardsand carrier frequencies are well within the broad scope of the presentinvention. The mixer 3056 adds the modulated signal to the carriersignal and provides a mixed signal to the second amplifier 3062. Thesecond amplifier 3062 is a variable gain amplifier whose output signalamplitude is determined by a gain control signal from the control andprocessing subsystem 3040. The gain control signal sets an output powerlevel of the signal from the second amplifier 3062. The signal from thesecond amplifier 3062 is provided to the first RFID sensing transmitconditioning filter 3031. The RFID sensing subsystem 3020 otherwiseoperates as set forth above with respect to FIG. 30. Alternatively, themodulated signal may be applied directly to an output amplifier by again control signal and thereby eliminate the need for the mixer 3056.

Turning now to FIG. 32, illustrated is a waveform diagram of anexemplary response from an RFID tag of an RFID object in accordance withthe principles of the present invention. The exemplary response includesrecorded transmissions 3215 and backscatter return signals 3220 from theRFID tag under docile conditions. Under docile conditions, the responsefrom the RFID tag is quite strong and substantially above the ambientnoise level 3210 and an interrogator can more readily detect theresponse on an individual bit-by-bit basis.

Turning now to FIG. 33, illustrated is a waveform diagram of a spectralresponse associated with the response from the RFID tag illustrated inFIG. 32. As illustrated, the spectral response provides a strong signalin accordance with the response from the RFID tag under docileconditions. The signal is essentially in two distinct components. Thefirst component is a strong backscatter return 3310, which is strongestin amplitude and at the center of the response. The second component isthe lower amplitude frequency shift keying (“FSK”) modulationbackscatter return 3315 consisting of a series of peaks. In hostileenvironments or, more generally, when the response from the RFID tag isnot as strong, such as when the RFID tag is located at an increasedrange from the interrogator or the RFID tag is obstructed from theinterrogator by absorptive or reflective materials, the backscatterreturn signals 3315 from the RFID tag to the interrogator aresubstantially weakened. Consequently, the detection and identificationof the RFID tag is much more difficult and an interrogator architecturethat can accommodate an improved signal to noise detection capabilityunder adverse conditions while not increasing the probability oferroneous responses would be advantageous. As will become more apparent,an interrogator and interrogation system constructed according to theprinciples of the present invention accommodates reliable identificationof the RFID tag under docile conditions and in hostile environments.

By way of example, consider a response from an RFID tag and theexistence thereof to be a one-bit message, namely, the RFID tag iseither present or not. Then, the presence of the RFID tag may be alogical “1” and an absence thereof may be a logical “0,” or vice versa.Then, further consider the bits of the reply code to be a spreading codefor the one-bit message. Spreading codes are used in spread spectrumcommunications to provide additional gain from signal processing forweak signals. For a better understanding of spread spectrum technology,see an “Introduction to Spread Spectrum Communications,” by Roger L.Peterson, et al., Prentice Hall Inc. (1995) and “Modern Communicationsand Spread Spectrum,” by George R. Cooper, et al., McGraw-Hill Book Inc.(1986), both of which are incorporated herein by reference.

Further assume that a reference code [representing a reply code orportions thereof such as a tag identification (“ID”) code] is preloadedinto an interrogator and the reply code from the RFID tag plus any noiseare correlated against the reference code by a correlation subsystemwithin the interrogator. If a match occurs, an increase in a gain [indecibels (“dB”)] for the matched signal within the interrogator followsthe relationships as set forth below:

Gain Increase(dB)=10×Log 10(N),

wherein “N” is the number of bits used in the correlation.

In a numerical example, if an RFID tag with a 64 bit tag ID code is usedfor the correlation, then the gain would be 18.06 dB. Additionally, ifan RFID tag with a 96 bit tag ID code and an eight bit preamble and 16bit cyclic redundancy check (“CRC”) is used for the correlation, thenthe gain would be 20.79 dB. The gain corresponds to an improvement inthe signal to noise ratio (“SNR”) as set forth above.

Turning now to FIG. 34, illustrated is a block diagram of portions of acontrol and processing subsystem of an interrogator constructedaccording to the principles of the present invention. The control andprocessing subsystem includes a correlation subsystem 3405 and adecision subsystem 3410. While in the illustrated embodiment thecorrelation subsystem and the decision subsystem form a portion of thecontrol and processing subsystem such as within a digital signalprocessor thereof, those skilled in the art should understand that thesubsystems may be discrete subsystems of the control and processingsubsystem of the interrogator or located in other locations of aninterrogation system.

The interrogator may employ a correlation operation to correlate betweenreference codes (generally designated 3415) corresponding to reply codes(generally designated 3425) from the RFID tags of RFID objects andsubsequently received and digitized reply codes from the RFID tags toenhance a sensitivity of the interrogator. The reply codes are typicallygenerated as complex I+jQ signals, where I signifies the in phaseportion of the signal and Q signifies its quadrature counterpart. Thereference codes may be scanned in during the initialization stage orderived synthetically as hereinafter described. To derive the referencecode synthetically, the amplitude, phase and delay (e.g., timing of aresponse to an excitation signal) information of a particular type ofRFID tag may be employed by the interrogator to derive the syntheticreference code. The correlation occurs in a correlator 3430 wherein thereply code is correlated (e.g., compared or matched) with the referencecode during a post-initialization stage of operation. The correlation ismathematically analogous to a convolution operation. For a betterunderstanding of convolution theory, see “An Introduction to StatisticalCommunication Theory,” by John B. Thomas, published by John Wiley &Sons, Inc. (1969), which is incorporated herein by reference.

A stream of incoming data in the form of a response to the interrogator(e.g., corresponding to responses in the form of reply codes from theRFID tags) is correlated against preloaded reference codes loaded into areference code database in time. Alternatively, samples of the incomingdata may be gated in a block by the interrogator and then the data iscorrelated in block manner against the reference codes. In the latterexample, a gating process is employed to gate the incoming dataproperly. Under such circumstances, apriori knowledge of a timing of theresponses from the RFID tag in connection with a query by theinterrogator better serves the process of gating the block of incomingdata (e.g., the responses) from the RFID tags. Any known delay in theresponses from the RFID tags can be preloaded in the interrogator duringthe initialization stage. An external sensor such as a position sensor(e.g., inertial sensor) may be employed by the interrogator to aid thecorrelation subsystem in predicting the timing of a response from theRFID tag. A synchronization pulse (e.g., derived from the transmitexcitation signal) may also be employed to better define a timing of aresponse from an RFID tag.

The output of the correlator 3430 representing individual correlationsof the reference code with incoming data is summed in a summer 3435providing a correlation signal to improve the signal to noise ratio ofthe correlated signal. The correlation signal from the summer 3435 istypically input into a threshold detector 3440 within the decisionsubsystem 3410 to verify a presence of an RFID tag. The thresholddetector 3440 typically compares the correlation signal with at leastone threshold criteria or value (also referred to as threshold). Thethreshold may be fixed or dynamically determined. In one exemplaryembodiment, where only a single threshold is present, an RFID tag isdeclared present if the correlation signal from the summer 3435 exceedsthe threshold, and not present if the converse is true. In otherembodiments, multiple thresholds may be used to indicate various levelsof probabilities as to the likelihood that an RFID tag is present ornot. This information may then be used to initiate selected oradditional search modes so as to reduce remaining ambiguities.

Regarding the timing of the responses from the RFID tag, a tracking ofthe reply codes may suggest that the reply code is early, prompt orlate. If the tracking suggests that the reply code is prompt (promptoutput greater than early and later output), then a gating function isproperly aligned to provide a significant correlator output. If thetracking suggests that the reply code is early, then the earlycorrelator output is significant as compared to the late correlatoroutput and the correlation subsystem 3405 is tracking too early and therequisite adjustment may be performed. An opposite adjustment may beperformed if the tracking suggests that the reply code is late.

Another approach is to use a tracking loop that uses past successfuldetection performance to establish a gating process for subsequentcorrelations. In yet another embodiment relating to the correlation ofthe reply codes from the RFID tags is to perform Fast Fourier Transforms(“FFTs”) on both the reference code and a gated sample of the replycodes from the RFID tags. Then, a convolution operation in “FourierSpace” may be performed employing the convolution theorem. Theconvolution theorem states that the convolution of two functions is theproduct of the Fourier transforms thereof. An output of the correlationoperation is typically envelope detected and several outputs may beaveraged in a summing operation that preserves time characteristics ofeach individual detection. For an example of such a control andprocessing subsystem, see U.S. Publication No. 2005/0201450 (the “450Publication), entitled “Interrogator and Interrogation System Employingthe Same,” to Volpi, et al., filed Mar. 3, 2005, which is incorporatedherein by reference.

Turning now to FIG. 35, illustrated is a block diagram of an embodimentof portions of a correlation subsystem associated with a control andprocessing subsystem of an interrogator demonstrating an exemplaryoperation thereof in accordance with the principles of the presentinvention. In the present embodiment, a technique referred to as a“corner turning memory” is used in accordance with the correlationsubsystem allowing a summing and averaging process for multiplecorrelations. An output of a correlator is read into memory by rows (oneof which is designated 3510) with each row designating a singlecorrelation. Then an output from the summing process (which embodies thememory or a function thereof) is generated by summing across individualcolumns (generally designated 3515, hence the name corner turning)applying an appropriate scaling factor. An output from the memoryrepresents an average of “P” outputs of the correlation subsystemwherein “P” is the number of rows in the corner turning memory. Assuminga signal is located in every row of the corner turning memory, theimprovement in signal to noise ratio (“SNR”) is increased by the squareroot of “P.”

Using this approach, several options for enhancing performance of theinterrogator are possible. For example, the results of differentaveraging times can be almost simultaneously compared and the modes ofoperation of the interrogator adjusted for enhanced performance. Also,this approach allows the sliding average technique (as described above)to be employed so that the output from the memory is an average over apredetermined period of time. Also, other averaging techniques inaddition to the use of the corner turning memory are well within thebroad scope of the present invention.

Turning now to FIG. 36, illustrated is a waveform diagram demonstratingexemplary advantages associated with the correlation subsystem describedwith respect to FIGS. 34 and 35. In the illustrated embodiment, aconventional waveform 3615 represents the probability of detection for agiven carrier to noise ratio (“C/No”) of a conventional reader readingat least one out of 100 possible attempts. A total of 100 trials wereaveraged in accordance with the correlation subsystem and an improvedwaveform 3610 represents the increased probability of detection for agiven C/No of an interrogator thereby demonstrating an improvement inSNR of 28.06 dB. This represents 18.06 dB due to correlation operationwherein a length 64 electronic product code (“EPC”) code was used, plusan additional 10 dB due to non-coherent averaging. A purpose of thecorrelation operation is to determine whether or not the output oraveraged output of the interrogator represents a presence of an RFIDtag. A threshold detector as herein described then interprets acorrelation signal from the correlation subsystem and provides adecision if the output is of sufficient quality to indicate if an RFIDtag is present or not and, if indeterminate, to perform a “deeper” ormore “focused” search.

Turning now to FIG. 37, illustrated is a waveform diagram demonstratingthe sidelobes associated with the correlation subsystem in accordancewith the principles of the present invention. Understanding the natureof the sidelobes and using their characteristics within a predetectingfunction can enhance the correlation subsystem of the interrogator. Asillustrated, the correlation includes a major peak 3710 (referred to as“prompt”) and two smaller peaks (generally referred to as “early” 3715and “late” 3720) about the major peak. By averaging the noise in theearly and late regions and comparing those values to noise levelsrecorded when it was known that no signal was present, additionalconfirmation is obtained that, in fact, an RFID tag is responding evenif the RFID tag is not uniquely identifiable in a single response at thepresent signal levels. Then, by averaging multiple responses thatcorrespond to RFID tag responses, the SNR will be raised to a levelwherein substantially unambiguous detection occurs.

In this instance, the reply code of an RFID tag is not being detected,but the interrogator is detecting a change in ambient noise thatsubstantially increases the probability that an RFID tag is indeedpresent. For example, sampling in all three regions and having the noiselevel be the same is a good indication that an RFID tag is not presentand therefore that the sample should be discarded. However, sampling inall three areas and finding that the early and late levels are aboutequal and the middle level is larger is a good indication that aresponse from an RFID tag is in fact present and that this sample shouldbe added into the averaging function. Clearly discarding samples that donot pass the early/late noise test will certainly discard data of actualRFID tags. That is a small price to pay, however, for not undulycorrupting the average with samples that do not in fact contain a replycode from an RFID tag. Sampling for slightly longer times compensatesfor the reduction in samples used. The control and processing subsystemcan maintain a running total of how many samples were discarded so thatthe number of samples averaged will remain valid.

Regarding the timing of the responses from the RFID tag, a tracking ofthe reply codes may suggest that the reply code is early, prompt orlate. If the tracking suggests that the reply code is prompt (promptoutput greater than early and late output), then a gating function isproperly aligned to provide a significant correlator output. If thetracking suggests that the reply code is early, then the earlycorrelator output is significant as compared to the late correlatoroutput and the correlation subsystem is tracking too early and therequisite adjustment may be performed. An opposite adjustment may beperformed if the tracking suggests that the reply code is late.

Another approach is to use a tracking loop that uses past successfuldetection performance to establish a gating process for subsequentcorrelations. In yet another embodiment relating to the correlation ofthe reply codes from the RFID tags, FFTs are performed on both thereference code and a gated sample of the reply codes from the RFID tags.Then, a convolution operation in “Fourier Space” may be performedemploying the convolution theorem. The convolution theorem states thatthe convolution of two functions is the product of the Fouriertransforms thereof. An output of the correlation operation is typicallyenvelope detected and several outputs may be averaged in a summingoperation that preserves time characteristics of each individualdetection.

Turning now to FIG. 38, illustrated is a block diagram of portions of anembodiment of a control and processing subsystem of an interrogatorconstructed according to the principles of the present invention. Thecorrelation subsystem of the control and processing subsystem correlatesusing multiple “a” values and then selects a correlation signal yieldingthe largest correlation value. The resultant is then run through aninitial constant false alarm rate (“CFAR”) subsystem whose principlefunction is to decide whether an RFID tag of an RFID object has fired.Secondarily, it is advantageous to filter out burst interference signalsfrom the correlation signals. The correlation signals that pass theinitial detection (“D-CFAR”) subsystem then enter a decision subsystem.

The next step is to align the correlation signals so the maximumresponse lag is at lag 0. An automatic gain control (“AGC”) correctionis also applied wherein the signal is normalized to have a peak power ofone (at lag 0) and then the other lags are normalized by the sameconstant. This has the effect of making the correlation signals have thesame weight in the subsequent steps.

The correlation signals are then envelope detected when employingnoncoherent multiple tag response integration. The result is passed intoan array of low pass filters (“LPF”). Finally, the LPF filter outputsare fed into a final detection CFAR (“D-CFAR”) subsystem that makes thefinal decision as to whether the RFID tag is present and anidentification CFAR (“I-CFAR”) subsystem that decides if the right RFIDtag is present, given a detection from the final D-CFAR subsystem.

Turning now to FIG. 39, illustrated is a waveform diagram demonstratingan application of a cell averaging constant false alarm rate with aninterrogator according to the principles of the present invention. Moreparticularly, the waveform diagram demonstrates an application of a cellaveraging CFAR to RFID detection wherein an analog to digital sampletiming is setup to produce 16 samples per bit. The temporal sidelobestructure seen is a consequence of the frequency shift keying (“FSK”)backscatter modulation used in autolD tags. Absent a strong correlationresponse, the central spike will not be seen.

A basic idea of the cell averaging CFAR is to average responses at lagvalues of {−64,−48,−32,−16, 32, 48, 64} with respect to the central 0lag point and use this as a threshold for testing the central lag forsignal presence. In CFAR parlance, the central cell is called the cellunder test (“CUT”). The average of the cells used to obtain thethreshold is called the cell average (“CA”).

Turning now to FIG. 40, illustrated is a waveform diagram of a cellunder test to a cell average ratio as a function of a cell under testlag using an unfiltered reference in accordance with a constant falsealarm rate in accordance with the principles of the present invention.Clearly, the ratio increases in the neighborhood of the central peak.The big ears at −6 & 8 lags are an artifact of the test signal usedbeing unfiltered and will tend to disappear with real signals. The aboveD-CFAR subsystem decides whether an RFID tag is present.

Turning now to FIG. 41, illustrated is a waveform diagram of a cellunder test to a cell average ratio for another constant false alarm ratein accordance with the principles of the present invention. In theillustrated embodiment, the current average lags are {−56, −40, −24, −8,8, 24, 40, 56}. These lags yield small correlation values when thesignal is present and so act more like a noise level reference. Again,this D-CFAR subsystem decides whether an RFID tag is present.

With continuing reference to FIG. 38, as gain is made larger; thethreshold is higher and thus the noise is rejected more effectively.This lowers the probability of false alarm, but it also lowersprobability of detection in the presence of the desired signal. Theinitial D-CFAR subsystem has a gain setup so that the correlation signalpasses, even if it is pretty weak, but at the cost of increased falsealarm rates. Occasionally, the correlation signal will pass in noise.The final D-CFAR subsystem has had the benefit of some filtering fromthe LPF array and so has its gain value tuned to a larger value so as tohave a low false alarm rate. When it declares a detection, the I-CFARsubsystem declares the right RFID tag.

As mentioned above, a corner turning memory is an important element inbuilding up SNR for weak signal detection purposes. Once the cornerturning memory is filled, an integrate and dump “filter” averages thecontents of each column of the corner turning memory and the resultantvector is presented to a CFAR subsystem for a detection decision asdescribed above. FIG. 42 illustrates an integrate and dump filter andits statistical properties. In the alternative, the integrate & dumpfilter can be replaced by a low pass filter as illustrated with respectto FIG. 43. The “1-A” multiplication provides a filter with unity DCgain.

From a noise perspective, the two approaches are statisticallyequivalent if:

${A = \frac{N - 1}{N + 1}},$

and

N Equivalent A 1 0.000000 3 0.500000 10 0.818182 30 0.935484 1000.980198The low pass filter approach has the advantage that it weights morerecent samples more heavily and gradually “forgets” about the oldersamples. To see this aspect, note how once a sample is inside the filterit re-circulates around the loop, each time being multiplied by A. AfterK iterations, the input value has been attenuated by a factor A^(K).

As a corollary to this, the low pass filter also has a charging time,prior to which, insufficient averaging has taken place. FIG. 44illustrates low pass filter transient responses for three different “N”values. As N increases, the filter becomes more sluggish in its responsebecause it is, in effect, averaging more samples. As a rule of thumb,the filter should be charged with N samples before accepting its outputas valid.

Inasmuch as the interrogator may be moving when detecting an RFID tag ofan RFID object, multiple time constant filters may be employed toadvantage. The short time constant filters (small N) would have fasterresponse, but less sensitivity, while longer time constant filters(larger N) would respond slowly to weak RFID tag responses, but wouldeventually respond. Each bank of filters (e.g., three banks having atime constant) would be followed by a CFAR subsystem and should providea unique output to an operator.

Turning now to FIG. 45, illustrated is an embodiment of a filterstructure for supporting multiple channels, with an indication of how itrelates to the D-CFARs and I-CFAR subsystems. Of particular note, oncealignment has occurred (see, e.g., alignment and AGC correction in FIG.38), extraneous cells that do not contribute to the decision process maybe discarded. Also, the lag structure is based on a presumption that theanalog to digital sample rates are tied to nominal 16 samples per databit. Again, it is preferable that the filter first be charged with Nsamples before using its output to drive the CFAR subsystems. Ifthroughput is an issue, the inputs the low pass filters can be operatedin series with decimation and yield essentially the same performance.Similarly, the medium and slow channel CFAR subsystems may not be runevery time a correlation signal passes the initial CFAR tests. Now thatexemplary control and processing systems and subsystems andinterrogation systems have been introduced, various systems andmethodologies will be introduced to enhance a sensitivity of theinterrogator and the interrogation systems.

It would be advantageous to the interrogation system to increase theeffective reading range in free space, and increase the ability to readRFID tags of an RFID object when an attenuating object is interposedbetween the RFID tag and the interrogator, of both passive andsemi-active RFID tags, by an approach to power management of thetransmitted signals from the interrogator. The system of powermanagement works with all interrogators, and is especially effectivewhen used in conjunction with interrogators employing correlators orcorrelation subsystems.

As an example, an interrogator can control the transmitted RF signalsuch that the amplitude of the signal may be varied under control of theinterrogator. The interrogator may increase the amplitude of any or allportions of the RFID tag interrogation sequence to deliver increasedenergy to the RFID tag to control the amount of energy delivered to theRFID tag before interrogation (e.g., allowing the RFID tag to storeenergy to be used during its response, also called “precharging”),during interrogation, or during the RFID tag's response tointerrogation. In so doing, the interrogator increases (e.g., maximizes)RFID tag detectability while at the same time reducing (e.g.,minimizing) the average amplitude of radiated energy.

In another aspect, an interrogator increases the time of unmodulated(also known as “continuous wave”) signal used to provide the energy toinitially activate the RFID tag from the minimum specified by standardsapplicable to the specific class of RFID tag being used. Currentindustry practice is to reduce the period of continuous wavetransmission to near the minimum required for standards compliance inorder to facilitate the rapid reading of RFID tags. The ability toextend the duration of the initial continuous wave period allows moretime for the RFID tag to accumulate energy for activation andbackscatter response. Additionally, an initial pilot tone return of theRFID tag can also be detected and aid in locating the presence of anRFID tag in weak signal conditions as provided above, even if theresponse is too weak to be completed or detected.

In another aspect, an interrogator allows precharging of the RFID tag tooccur by sending a sequence of messages, with no intervening time delay,to which the RFID tag cannot respond, followed immediately by aninterrogation command (e.g., a single interrogation command). Theinitial sequence of messages will result in a relatively long period inwhich energy is presented to the RFID tag while the RFID tag is notrequired to expend energy to respond, resulting in energy accumulationwithin the RFID tag. The single interrogation command that followscauses the RFID tag to expend the energy in a single response.

Thus, the interrogator is managing power in an intelligent way in orderto get more performance out of the interrogation system while stillmaintaining full standards compatibility with whatever type of RFID tagis used and while still being fully compliant with any and all maximumtransmit power specifications so long as those specifications aredefined over times that are long with respect to a singleinterrogation/reply sequence. For a better understanding of RFID tags,see “Technical Report 860 MHz-930 MHz Class I Radio FrequencyIdentification Tag Radio Frequency & Logical Communication InterfaceSpecification Candidate Recommendation,” Version 1.0.1, November 2002,promulgated by the Auto-ID Center, Massachusetts Institute ofTechnology, 77 Massachusetts Avenue, Bldg 3-449, Cambridge, Mass.02139-4307, and “EPC Radio-Frequency Identity Protocols Class-1Generation 2-2 UHF RFID Protocol for Communications at 860-960 MHz,”Version 1.09, January 2005, promulgated by EPCglobal Inc., PrincetonPike Corporate Center, 1009 Lenox Drive, Suite 202, Lawrenceville, N.J.08648, which are incorporated herein by reference.

A protocol independent interrogation system (e.g. an RFID interrogationsystem) is described that includes at least one RFID excitation source,typically embodied in a transmit function, and a corresponding RFIDreceive function. The transmit and receive functions may be employed inan interrogator that includes control and processing subsystems andsensing subsystems embodied in a software defined architecture wherein asignificant portion of the signal processing is done in the digitaldomain after an incoming signal plus any associated noise has beenappropriately digitized. The interrogator can deliver power to the RFIDtag to permit the RFID tag to fully or partially respond to excitationunder conditions of attenuation of the transmitter signal that precludeoperation of presently available readers. This enhanced ability toexcite the RFID tag has applications in both extending the usefuldetection range for RFID tags in free space, and in detecting RFID tagswhen signal attenuating objects are present between the RFID tag and theinterrogator.

Turning now to FIGS. 46 and 47, illustrated are diagrams of aninterrogation sequence in accordance with an interrogator. Theinterrogation sequence contains a period in which no energy istransmitted (designated “RF OFF”), followed by a period in which anunmodulated or continuous wave message (“CW”) is transmitted (designated“CW Period”), followed by known message patterns wherein modulation isadded for use by a receiving RFID tag to first synchronize a clock ofthe RFID tag with the interrogator (designated “Data Modulation”),followed by the message content and message integrity controlinformation (e.g., checksums or cyclic redundancy check codes,designated “Setup Phase”). Upon completion of the transmitted command,the interrogator continues to send continuous wave energy as anunmodulated CW message (designated “Unmodulated CW”). It is this energythat is then modulated via impedance matching/mismatching (also known as“backscatter”) by the RFID tag. The interrogator then detects thismodulated energy, and decodes the information sent by the RFID tag.

The duration of the initial no-transmission and continuous wave periods(collectively known as the “preamble”) each typically have minimumdurations defined by applicable standards. Due to the requirementsmentioned above of typically being able to read an RFID tag as quicklyas possible, and also of reading as many RFID tags within a given timeperiod as possible, current commercial practice is to maintain both theno-transmission and continuous wave periods near the minimums specifiedby the standards. The interrogation system as provided herein may makeuse of increasing the aforementioned time periods to achieve greatersensitivity in detecting the presence of RFID tags.

As illustrated in FIG. 47, an interrogator is attempting to detect apassive (no onboard power) RFID tag. The region designated “RF Energy”illustrates that portion of the timeline when RF energy is beingtransmitted by the interrogator. Often a situation exists whensignificant signal attenuation may exist between the interrogator andthe RFID tag so that the energy necessary to activate the RFID tag isinsufficient or the energy reflected back to the interrogator is beyondthe sensitivity thereof causing an RFID tag to be undetected. Thisattenuation may be due to a large distance in free space between theinterrogator and the RFID tag, or may be due to an adverse environmentproviding significant attenuating characteristics.

In either case, under these conditions, the total energy received by theRFID tag during the continuous wave message period (designated “CWPeriod”) of the preamble may be insufficient to adequately charge theRFID tag and allow the RFID tag to even begin to operate or,correspondingly, the RFID tag may have sufficient energy to beginoperation, but be unable to complete its transmission because ofinsufficient energy. Experience with RFID tags has determined thatenergy requirements necessary to modulate the signal, by impedancematching/mismatching, are relatively high as compared to the energyrequirements for the RFID tag's onboard processor to operate. Thus,there exists a range of conditions in which sufficient energy isavailable for the RFID tag to begin transmission, but not complete thetransmission and fully respond during the unmodulated CW period(designated “Unmodulated CW”). The result is system failure as regardsto detecting and identifying that RFID tag.

Turning now to FIG. 48, illustrated is an embodiment of an interrogationsequence in accordance with an interrogator constructed according to theprinciples of the present invention, which will increase the energyavailable to the RFID tag via a method referred to as “precharging.” Inthis case, the period of the continuous wave message (designated “CWPeriod”) in the preamble has been extended as compared to the intervalused in current commercial practices. For example, the period of thecontinuous wave message may be extended by at least one millisecond.Based upon experience with RFID tags, it has been observed that the RFIDtags exhibit the ability to accumulate energy over a period of time. Theinterrogation sequence shown extends the period of continuous wavetransmission, allowing additional time for the RFID tag to accumulateenough energy to power itself, begin transmission and possibly tocomplete the transmission. As described earlier, even a partialtransmission may be useful in some instances.

Additionally, as illustrated in FIG. 48, the average amplitude alsovaries with time with the periods representing the continuous wavemessage (designated “CW Period”), the data modulation message(designated “Data Modulation”) and the setup phase message (designated“Setup Phase”) having a larger average amplitude (designated “A1”) thanthe period representing the unmodulated CW period (designated“Unmodulated CW”) with an average amplitude (designated “A2”). In sodoing, additional energy is incident on the RFID tag, increasing thelikelihood of the RFID tag having adequate energy to decode themodulated data from an interrogator and then respond during the periodassociated with the unmodulated CW message. By reducing the power, theaverage power over the entire process is also reduced. An interrogatorsuch as that described in the '450 Publication due to its increasedsensitivity would be able to read the modulated reflected energy fromthe RFID tag during the interrogator's unmodulated CW transmission. Thisregime is, by definition, of relatively short duration, thereby allowingthe interrogator to comply with aggregate power emission limitations.

Alternatively, it is not necessary to reduce the power and maintain theincreased level throughout the entire process. Additional embodimentsconsisting of various other combinations of high and low power periodsto augment RFID tag precharging while maintaining low average power arecomprehended within the context of this invention.

Yet another embodiment for precharge capability according to theprinciples of the present invention is to send, as rapidly as possible,a series of messages within the interrogation sequence that the RFID tagcannot respond to, followed immediately by an interrogation command.This method results in a situation wherein the RFID tag has an extendedopportunity to accumulate energy prior to being required to transmit,although it is not as effective as the extended continuous wave methoddescribed above due to the periods of no transmission contained withinthe preambles on each of the individual messages. The advantage of thismethod is that it is easily implemented on many existing interrogators.

Thus, a sensing subsystem of the interrogator transmits an unmodulatedcontinuous wave message and detects a modulated version of thecontinuous wave message from the RFID tag. A control and processingsubsystem of the interrogator discerns a presence of the RFID tag fromthe modulated version of the continuous wave message and decodesinformation from the RFID tag. The sensing subsystem is configured tovary instantaneous power of an excitation signal to an RFID tag to varyan energy incident on the RFID tag. The control and processing subsystemis configured to control the sensing subsystem to vary the instantaneouspower as a function of time periods within an interrogation cycle orsequence and to maintain an average power thereof below a predeterminedvalue. There may be a series of actions wherein the interrogatorenergizes the RFID tag and receives a response therefrom in a singleinterrogation. There also may be a series of actions wherein theinterrogator modulates and unmodulates a signal to the RFID tag beforegetting a response therefrom or a period of time wherein no RF energy istransmitted, then the RFID tag is charged, followed by a command fromthe interrogator to setup and listen.

For a better understanding of communication theory and radio frequencyidentification communication systems, see the following references “RFIDHandbook,” by Klaus Finkenzeller, published by John Wiley & Sons, Ltd.,2nd edition (2003), “Introduction to Spread Spectrum Communications,” byRoger L. Peterson, et al., Prentice Hall Inc. (1995), “ModernCommunications and Spread Spectrum,” by George R. Cooper, et al.,McGraw-Hill Book Inc. (1986), “An Introduction to StatisticalCommunication Theory,” by John B. Thomas, published by John Wiley &Sons, Ltd. (1995), “Wireless Communications, Principles and Practice,”by Theodore S. Rappaport, published by Prentice Hall Inc. (1996), “TheComprehensive Guide to Wireless Technologies,” by Lawrence Harte, et al,published by APDG Publishing (1998), “Introduction to Wireless LocalLoop,” by William Webb, published by Artech Home Publishers (1998),“Digital Communications,” by John C. Proakis, 3rd Edition, McGraw-Hill,Inc. (1995), “Antenna Engineering Handbook,” by Richard Johnson andHenry Jasik, McGraw-Hill, Inc. (1992), “Wideband Wireless DigitalCommunications,” by Andreas F. Molisch, Pearson Education (2000), and“The Mobile Communications Handbook,” by Jerry D. Gibson, published byCRC Press in cooperation with IEEE Press (1996). For a betterunderstanding of conventional readers, see the following readers,namely, an “MP9320 UHF Long-Range Reader” provided by SAMSysTechnologies, Inc. of Ontario, Canada, an “MR-1824 Sentinel-Prox MediumRange Reader” by Applied Wireless ID of Monsey, N.Y. (see also U.S. Pat.No. 5,594,384 entitled “Enhanced Peak Detector,” U.S. Pat. No. 6,377,176entitled “Metal Compensated Radio Frequency Identification Reader,” andU.S. Pat. No. 6,307,517 entitled “Metal Compensated Radio FrequencyIdentification Reader”), “2100 UAP Reader,” provided by IntermecTechnologies Corporation of Everett, Wash. and “ALR-9780 Reader,”provided by Alien Technology Corporation of Morgan Hill, Calif. Theaforementioned references, and all references herein, are incorporatedherein by reference in their entirety.

Thus, an interrogator, an interrogation system and method of operatingthe same have been introduced herein. In an aspect, the interrogatorincludes an RFID sensing subsystem configured to detect an RFID object,and a control and processing subsystem configured to control aninterrogation sequence of the RFID sensing subsystem by precharging theRFID object prior to detecting the RFID object.

The control and processing subsystem can control the interrogationsequence in many ways. For instance, the control and processingsubsystem may vary an amplitude of a portion of the interrogationsequence. The control and processing subsystem is also configured toextend a duration of a continuous wave message of the interrogationsequence in accordance with precharging the RFID object.

In a related aspect, the interrogation sequence includes a continuouswave message, a data modulation message, a setup phase message and anunmodulated continuous wave message. In accordance therewith, thecontinuous wave message, the data modulation message and a setup phasemessage may have a first amplitude and the unmodulated continuous wavemessage may have a second amplitude.

In another related aspect, the RFID sensing subsystem is configured totransmit an unmodulated continuous wave message and detect a modulatedversion of the continuous wave message from the RFID object. Inaccordance therewith, the control and processing subsystem is configuredto discern a presence of the RFID object from the modulated version ofthe continuous wave message and decode information from the RFID object.

Further refinements may also be provided to the interrogation systems asintroduced herein. For instance, a method is described to increase thedetection sensitivity of an interrogator such as described in the '450Publication by an approach dealing with clock frequency errors in theresponses transmitted by RFID tags. Passive and semi-active RFID tagsoperate on the principle of an interrogator sending out interrogatingsignals or an interrogation command by modulating an RF carrier and thenreceiving a response from an RFID tag by having that RFID tag modulateits backscatter characteristics in a controlled manner. In so doing, aunique modulated response is sent back to the interrogator where it isdetected and decoded. Due to small size and low cost, interrogationsystems like this are desirable in many applications including supplychain management, inventory control, and the general counting of and/oraccounting for items in various industries and market segments.

Recent advancements in interrogators such as disclosed in the '450Publication have greatly improved the ability of a sensitivity of theinterrogator for such a passive or semi-active system's ability to beable to accurately and reliably receive and decode a very weak RFID tagto interrogator (also referred to as “Tag-to-Reader”) signal. However,experience with commercially-available RFID tags has shown that the RFIDtags sometimes transmit their response to an interrogation using a clockfrequency that deviates from that specified by the interrogator.Correlation-based detectors, such as described in the '450 Publication,are acutely sensitive to deviations from the expected response clockrate and lose sensitivity when the RFID tag's response clock ratediffers from the expected rate. Therefore, what is needed is an improvedmethod for constructing a correlator that is tolerant of RFID tagresponse clock frequency variations.

In one aspect, an interrogator constructed according to the principlesof the present invention is able to apply the correlation andnon-coherent integration mechanisms described in the '450 Publication insituations in which the RFID tag response to the interrogator does notoccur at the clock frequency specified by the interrogator bycorrelating not only against a synthetic tag signature with the correctclock, but also correlating against several synthetic tag signatureswith clock errors approximating typical RFID tag response clock errors.The results of these parallel correlations are then compared, and thebest response is selected. The process of creating a synthetic tagresponse signature with a clock slower or faster than normal is called“signature scaling.” Use of signature scaling in the correlation andintegration of these previously unusable RFID tag responses results in asubstantial increase in interrogator sensitivity.

Turning now to FIG. 49, illustrated are waveform diagrams of anembodiment of an interrogation sequence from an interrogator, along withtwo RFID tag response waveforms from RFID tags designated Tag A and TagB in accordance with the principles of the present invention. As part ofthe interrogation sequence, an interrogator sends a known sequence ofpulses intended to allow the RFID tag to synchronize its internal clockand thereby transmit its response at the frequency desired by theinterrogator (the waveform shows a portion of a typical timing componentof an interrogation request). The response waveforms for Tag A and Tag Bshow that the RFID tags are quiescent, but are receiving the timingpulses and synchronizing their internal clocks during this time. In thisexample, Tag A is assumed to have correctly set its internal clock bymeasuring the frequency of the timing pulses from the interrogator, butTag B has derived a frequency with an approximate one percent error.Though a one percent error is not obvious in the first part of thisFIGURE, its consequences are readily revealed later in time.

After transmission of the interrogation sequence, the interrogatorenters a mode in which it transmits a continuous wave at the RFID tagmodulation frequency to provide both power for the RFID tag and a signalthat the RFID tag may modulate via backscattering. The section of thewaveform for the interrogation sequence designated 4920 shows thiscontinuous wave transmission. After an interval specified by theapplicable RFID tag technology standard, the RFID tag beginstransmitting its response, again, by modulating (e.g., backscattering)the continuous wave being transmitted by the interrogator. During theinitial part of the response, no difference is discernable at the scaleof this drawing between the response of Tag A as shown at 4925 and Tag Bas shown at 4930.

By the end of response transmission, however, a difference isdiscernable between the response of Tag A at 4935 and that of Tag B at4940. Due to the approximately one percent error in determining theproper response frequency, the response from Tag B has taken about onebit-time longer than the correct response of Tag A. The differencebetween the correct end of transmission time, tO, and the incorrect endof transmission time, t1, is the absolute error in the RFID tag responseclock. This value, when divided by the transmission time of the completeRFID tag response using a correct clock, yields the clock error ratio.

Experience with RFID tags has shown that the response clock measurementsare inaccurate in a significant percentage of RFID tag responses. Theinterrogator described in the '450 Publication provides a significantimprovement in detection of RFID tags by, among other means, usingcorrelation against a synthetic RFID tag signature. Correlation isacutely sensitive to clock rate between the synthetic tag signature andthe received tag response, consequently correlation fails or exhibitsdecreased sensitivity in those cases in which the RFID tag responds at aclock frequency significantly different than that specified by theinterrogator.

Turning now to FIG. 50, illustrated are waveform diagrams of anembodiment of an interrogation sequence from an interrogator, responsefor an RFID tag, and correlation signals from a correlation subsystem ofan interrogator in accordance with the principles of the presentinvention employing multiple correlators. In this embodiment,correlation is attempted between the synthetic reference code or signalat the expected clock rate and the received signal, as per the '450Publication, but correlation is also attempted between syntheticreference codes, signals or waveforms generated with clock rates varyingfrom the expected clock rate. Experience with RFID tags has shown thatresponse clock frequency errors are bounded, and that a relatively smallnumber of clock rate variant synthetic waveforms (and consequently asmall number of additional correlations) are advantageous to detect themajority of RFID tag clocking errors. The response received from an RFIDtag (using a clock rate approximately 0.5 percent slow in this case) isshown in the received RFID tag response. The synthetic waveform fastshows a synthetic signature with approximately a −0.5 percent error. Thesynthetic waveform correct shows a synthetic signature with a correctclock, and the synthetic waveform slow shows a synthetic signature withapproximately a +0.5 percent error. The timing marks designated 5025illustrate the relative differences in clock rates.

Per the '450 Publication, the RFID tag response waveform is correlatedagainst a synthetic waveform at the correct clock rate, with the resultshown at 5030. However, parallel correlations are also performed againstsynthetic tag signatures with a slightly fast clock, at 5035, and aslightly slow clock, at 5040. Note that the correlation result from theslow clock displays a valid correlation triangle, while the correlationresults from the fast and correct synthetic waveforms display amalformation characteristic of clock errors in which two temporallyseparated correlation triangles appear to be overlaid, forming acharacteristic “bat ears” shape. In this case, the correlation resultsare examined using methods described in the '450 Publication, resultingin positive detection despite the error in the RFID tag's response.

Thus, a control and processing system or subsystem for an interrogator,an interrogation system, and a method of verifying a presence of an RFIDobject has been introduced herein. In one aspect, the control andprocessing system includes a memory configured to store a reference code(e.g., a synthetically derived reference code). The control andprocessing system includes a correlation subsystem configured tocorrelate the reference code at a first clock rate and a second clockrate with a reply code from an RFID object and provide correlationsignals therefrom. The correlation subsystem may include multiplecorrelators. A decision subsystem of the control and processing systemis configured to verify a presence of the RFID object as a function ofthe correlation signals. Regarding an operation of the control andprocessing system, the reference code may be provided during aninitialization stage of operation and the reply code may be providedduring a post-initialization stage of operation.

In one aspect, the correlation subsystem is configured to correlate in atime domain employing an exclusive OR function or correlate employing aFast Fourier Transform and a convolution theorem. In another aspect, thecorrelation subsystem includes a correlator configured to correlate atleast two bits of the reference code with at least two bits of the replycode to derive correlation triangles. A correlation threshold sense ofthe correlation subsystem is configured to compare the correlationtriangles to a threshold criteria to derive pulses to ascertain peaks ofthe correlation triangles. A summer of the correlation subsystem isconfigured to average a plurality of pulses from the correlationthreshold sense to provide the correlation signals.

In another aspect, the decision subsystem includes a threshold detectorconfigured to compare the correlation signals to a threshold. Thedecision subsystem is also configured to verify the presence of the RFIDobject by employing a statistical analysis on a result therefrom.

To further refine a sensitivity of an interrogation system, a method isproposed to detect the presence of an RFID tag within the interrogationfield in cases in which the RFID tag does not transmit a full responseto an interrogation. Radio frequency identification is one of thefastest growing areas within the field of automatic identification anddata collection. A reason for the proliferation of RFID systems is thatRFID tags may be affixed to a variety of diverse objects (also referredto as “RFID objects”) and a presence of the RFID tags may be detectedwithout actually physically viewing or contacting the RFID tag. As aresult, multiple applications have been developed for the RFID systemsand more are being developed every day.

The parameters for the applications of the RFID systems vary widely, butcan generally be divided into three significant categories. First, anability to read the RFID tags rapidly. Another category revolves aroundan ability to read a significant number of the RFID tags simultaneously(or nearly simultaneously). A third category stems from an ability toread the RFID tags reliably at increased ranges or under conditionswherein the radio frequency signals have been substantially attenuatedor distorted, or in environments in which there is a substantial amountof ambient radio frequency noise or interference occurring within thefrequency range used by the interrogator and tags.

While significant progress has been made in the area of reading multipleRFID tags almost simultaneously (see, for instance, U.S. Pat. No.6,265,962 entitled “Method for Resolving Signal Collisions BetweenMultiple RFID Transponders in a Field,” to Black, et. al., issued Jul.24, 2001, which is incorporated herein by reference), there is stillsubstantial room for significant improvement in the area of reading theRFID tags reliably at increased ranges, or under conditions when theradio frequency signals have been substantially attenuated, or inenvironments in which a substantial amount of ambient radio frequencynoise or interference exists within the frequency range used by theinterrogator and RFID tags. In some environments, the energy transmittedby the interrogator is attenuated to the extent that insufficient energyexists for an RFID tag to complete transmission, yet detection of apartial transmission provides definitive evidence of the presence of anRFID tag in the interrogation field and, therefore, valuable informationin certain applications. Therefore, what is needed is a method toreliably detect partial transmissions from RFID tags.

In one aspect, an interrogator constructed according to the principlesof the present invention is able to detect the presence of an RFID tagwithin the interrogation field even if the RFID tag has insufficientpower to transmit a complete response. Experience with RFID tags hasshown that the energy required to activate an RFID tag is relatively lowas compared to the energy required for the RFID tag to backscatterenergy from the interrogator by switching impedances on the RFID tag'santenna. Thus, in situations wherein energy from the transmitter isheavily attenuated, an RFID tag may receive sufficient energy toactivate and begin transmitting, but the act of transmitting soonexhausts all available energy and the RFID tag deactivates before thetransmission is complete. Thus, the partial tag integration capabilitydescribed in the '450 Publication is extended, thereby allowing theinterrogator to correlate as little as only the first few bits of theRFID tag's response (typically a common value among a class of RFIDtags) against a synthetic signature (see the '450 Publication) anddetermine, to a high degree of reliability, the presence of an RFID tag.

Turning now to FIG. 51, illustrated are waveform diagrams of anembodiment of full RFID tag responses and a partial RFID tag response inaccordance with the principles of the present invention. As describedabove, partial RFID tag responses are common when the interrogator'ssignal is heavily attenuated, resulting in insufficient energy reachingthe RFID tag for a complete response. In such cases, the RFID tagfrequently will begin transmission, only to fail and cease operatingduring the transmission due to exhaustion of stored energy. The fullRFID tag response waveform 5105 shows a complete response from an RFIDtag, while the partial RFID tag response waveform 5110 shows a partialresponse, in which the RFID tag had insufficient power to continuetransmitting after the 8th bit of the preamble.

In certain situations it is useful to determine if an RFID tag existswithin the interrogation field, even if the RFID tag's informationcannot be completely read. Using the correlation method as described inthe '450 Publication, it is possible to correlate against the RFID tagresponse preamble (e.g., may include a pilot tone along with a fixednumber of bits independent of the RFID tag's identification), which iscommon to RFID tags of a specific type. The partial correlation resultwhen RFID tag present waveform 5115 shows the characteristic result ofcorrelating the eight bit response in accordance with the partial tagresponse waveform 5110 with an eight bit synthetic tag signature usingthe methods described in the '450 Publication. The partial correlationresult when no RFID tag present waveform 5120 shows the results of thesame type of correlation when no RFID tag response was received.Additionally, it may be advantageous to perform correlation on the pilottone wherein the RFID tag modulates the carrier by a fixed constantfrequency for a period of time before sending any specifically encodeddata bits.

Inspection of the results of partial correlation, using techniquesmeasuring, among other factors, symmetry, the monotonicity, and the peakspacing, are sufficient to reliably differentiate the two RFID tagresponse waveforms 5115, 5120 and detect the presence of a partiallyfiring RFID tag to a high degree of accuracy. Even correlating only aneight bit preamble as illustrated will provide a nine decibel increasein sensitivity when an RFID tag is present, and substantially increasesthe probability that the presence of an RFID tag will be detected withinthe interrogation field. Additional detection enhancement is possibleshould more or other portions of the backscatter waveform be availablefor processing, which is comprehended by this invention.

Additionally, multiple correlators, as discussed above in signaturescaling, may also be employed to further enhance the sensitivity ofpartial tag response detection. This is because as an RFID tag no longerhas adequate energy to continue proper response to an inquiry, it maynevertheless continue for several more cycles. The frequency of thoselast cycles, however, will likely differ sufficiently from the initialcycles so that they will not contribute to the gain improvement due tocorrelation. Multiple correlators as discussed above in signaturescaling along with the inclusion of additional correlators so as tocover a broader frequency range will capture those final cycles. Then,because in this instance multiple responses may exist, systems thatcombine the responses of multiple correlators can be used to increasethe strength of the detected response. As an example, the simplenoncoherent addition of all responses will provide a stronger response.The simple example above is only meant to illustrate the concepts ofthis invention and other methods are certainly possible using this data,and this invention comprehends them as well.

Systems and methods are introduced to both increase the detectionsensitivity and the discrimination capability of a correlation-baseddetector as described in the '450 Publication by an approach thatcorrelates a received signal against a synthetic signal containingeither only the clock waveform of the RFID tag, or the data waveform ofthe RFID tag. As mentioned above, passive and semi-active RFID tagsoperate on the principle of an interrogator sending out interrogatingsignals by modulating an RF carrier and then receiving a response froman RFID tag by having that RFID tag modulate its backscattercharacteristics in a controlled manner. In so doing, a unique modulatedresponse is sent back to the interrogator or reader to be detected anddecoded. Due to their small size and low cost, systems like this aredesirable in many applications, including supply chain management,inventory control, and the general counting of and/or accounting foritems in various industries and market segments.

Additionally, recent advancements in interrogator architecture such asdisclosed in the '450 Publication have greatly improved the ability ofan interrogator's sensitivity for such a passive or semi-active system'sability to be able to accurately and reliably receive and decode a veryweak Tag-to-Reader signal. The attenuation and radio frequencynoise/interference in some environments may be so severe that attemptingcorrelation with a synthetic signature for a specific RFID tag'sinformational content may not be feasible due to decreased sensitivity,and certain applications exist in which it is valuable to detect thepresence of an RFID tag within the interrogation field even when it isnot possible to uniquely identify that RFID tag through reception of itscomplete identification by the interrogator. Other applications exist inwhich it is necessary to distinguish between the signatures of two RFIDtags with substantially identical data contents under conditions ofattenuation and radio frequency noise/interference. What is needed,therefore, is a system of correlating with a synthetic tag signaturethat provides a high detection value for any RFID tag within theinterrogator's field, and a method of improving discriminationcapability for the RFID tags having substantially identical data values.

In one aspect, an interrogator constructed according to the principlesof the present invention is able to detect the presence of any RFID tagin the interrogation field, with a single interrogation command, underconditions in which the complete RFID tag response is unreadable due toattenuation or radio frequency noise or other interference, by detectingand correlating on the presence of the clock information signal that ispart of every RFID tag response using correlation techniques taught inthe '450 Publication regardless of information content. Two advantagesaccrue through the use of this mechanism. First, it provides anefficient method of determining if any RFID tag within a specific airinterface standard exists within the interrogation field, regardless ofRFID tag encoding. Second, it provides a method of determining if anRFID tag exists within the interrogation field even if attenuation ornoise or other effects are such that the information content of the RFIDtag response cannot be detected.

In another aspect, an interrogator constructed according to theprinciples of the present invention is able to discriminate between thesignals of two RFID tags with substantially identical data values bydetecting and correlating on the presence of the data component orinformation of the RFID tag response using correlation techniques taughtin the '450 Publication. This method provides the advantage ofadditional detection accuracy for situations in which similarly numberedRFID tags should be detected to a very high degree of accuracy.

Turning now to FIG. 52, illustrated are waveform diagrams of anembodiment for a string of modulated binary zeros (waveform designated5205) and for a string of modulated binary ones (waveform designated5210) as encoded using frequency shift keying in accordance with theprinciples of the present invention. In this example, a binary zero hasa single zero-crossing, while a binary one has two zero crossings.Providing that there be at least one crossing per binary data bitensures that the receiver (e.g., an RFID sensing subsystem within theinterrogator) of this waveform can correctly decode it. Thus, one ofthese zero crossings is common to both zero and one symbols, andfunctions as an embedded timing reference, or “clock” for the receiver.Since each symbol has a common transition, regardless of whether a oneor zero is being transmitted, there is a great deal of information incommon between the signals transmitted by two different RFID tags, evenif the data values within a reply code of the RFID tags themselves arecompletely different. This characteristic of an RFID tag responseencoding can be exploited to improve both reception sensitivity andreception discrimination. The waveform designated 5215 shows averagingthe modulated one and zero waveforms above, hereinafter known as the“clock” waveform.

Turning now to FIG. 53, illustrated are waveform diagrams of anembodiment of reference codes or RFID tag signatures including data andclock signals or information (waveform designated 5305), clock-onlysignals or information (waveform designated 5310), and data-only signalsor information (waveform designated 5315) in accordance with theprinciples of the present invention. A synthetically derived data andclock reference code may used for correlation as in the '450 Publicationand provides a balance between the ability to detect an RFID tag and theability to discriminate between two RFID tags with substantially similardata information.

Correlating with the clock-only information will produce a positiveresult if any RFID tag within an entire class of RFID tags responds, andwill produce a usable correlation result under conditions of attenuationand interference that would otherwise preclude correlation for aspecific RFID tag data value. Thus, clock-only correlation providesbenefits in situations such as described in the '450 Publication toconfirm, for instance, that no RFID tag exists within the interrogator'sfield, as well as in other similar situations.

Correlating with the data-only information correlates the received replycode or portion thereof against a signal that has had the common clockinformation removed for the reference code. This has the effect ofincreasing the difference in correlation results between two RFID tagswith similar data information, since the clock information no longercontributes to the correlation result. Data-only correlation providesbenefits in situations to discriminate, for instance, between two ormore RIFD tags with similar data information.

A method is proposed to improve the performance of non-coherentintegration correlation based detection systems, such as described inthe '450 Publication, by analyzing the energy distribution within thecorrelation results to generate a probability of detection value thatcan be used by other statistical methods to detect or identify RFID tagswith a high degree of certainty. As mentioned above, passive andsemi-active RFID tags operate on the principle of an interrogatorsending out interrogating signals by modulating an RF carrier and thenreceiving a response from an RFID tag by having that RFID tag modulateits backscatter characteristics in a controlled manner. In so doing, aunique modulated response is sent back to the interrogator or readerwherein it is detected and decoded. Due to their small size and lowcost, systems like this are desirable in many applications includingsupply chain management, inventory control, and the general counting ofand/or accounting for items in various industries and market segments.

Thus, an interrogator, an interrogation system, and a method ofverifying a presence of an RFID object has been introduced herein. Inone aspect, the interrogator includes an RFID sensing subsystemconfigured to receive a partial response from an RFID object including aportion of a reply code. The interrogator also includes a control andprocessing subsystem including a correlation subsystem configured tocorrelate a portion of a reference code with the portion of the replycode and provide a correlation signal therefrom. The control andprocessing subsystem also includes a decision subsystem configured toverify a presence of the RFID object as a function of the correlationsignal.

In a related aspect, the portion of the reply code may include clock ordata information and the portion of the reference code may includesynthetically derived clock or data information. In accordancetherewith, the correlation subsystem is configured to correlate thesynthetically derived clock or data information with the clock or datainformation of the reply code and provide a correlation signaltherefrom. Regarding an operation of the interrogator, the referencecode may be provided during an initialization stage of operation and theportion of the reply code may be provided during a post-initializationstage of operation.

In one aspect, the correlation subsystem includes multiple correlatorsand the correlation subsystem is configured to correlate in a timedomain employing an exclusive OR function or employing a Fast FourierTransform and a convolution theorem. In another aspect, the correlationsubsystem includes a correlator configured to correlate at least twobits of the reference code with at least two bits of the reply code toderive a correlation triangle. A correlation threshold sense of thecorrelation subsystem is configured to compare the correlation triangleto a threshold criteria to derive a pulse to ascertain a peak of thecorrelation triangle. A summer of the correlation subsystem isconfigured to average a plurality of pulses from the correlationthreshold sense to provide the correlation signal. The correlationsubsystem may also be configured to employ multiple amplitude bits ofthe portion of the reference code and the portion of the reply code.

In another aspect, the decision subsystem includes a threshold detectorconfigured to compare the correlation signal to a threshold. Thedecision subsystem may also be configured to verify the presence of theRFID object by employing a statistical analysis on a result therefrom.

Recent advancements in RFID interrogation architecture such as disclosedin the '450 Publication have greatly improved the ability of asensitivity of the interrogator for such a passive or semi-activesystem's ability to be able to accurately and reliably receive anddecode a very weak Tag-to-Reader signal. The attenuation and radiofrequency noise/interference in some environments may be so severe thatthe results of a single correlation, or a small number of correlations,are inadequate to declare, with a high degree of certainty, the presenceor absence of any RFID tag, or of a specific RFID tag. What is needed,therefore, is a means for analyzing the correlation results between asampled waveform and a synthetically derived reference code or signatureand to determine a detection probability that can be used by otherstatistical processing mechanisms to detect the presence of an RFID tagwith a high degree of certainty, or to identify a specific RFID tag witha high degree of certainty.

In one aspect, an interrogator constructed according to the principlesof the present invention can detect the presence of any RFID tag withinthe interrogation field under highly attenuative or high ambient radiofrequency noise/interference conditions by examination of the energydistribution within the correlation response (see, e.g., the '450Publication). Using this method in conjunction with the correlation andintegration methods described in the '450 Publication allows statisticalmethods to be used to determine the presence and/or identity of an RFIDtag within the interrogation field to a high degree of certainty.

In another aspect, an interrogator constructed according to theprinciples of the present invention can discriminate between theresponses of RFID tags with similar informational content within theinterrogation field under highly attenuative or high ambient radiofrequency noise/interference conditions by examination of the energydistribution within the correlation response (see, e.g., the '450Publication). Using this method in conjunction with the correlation andintegration methods described in the '450 Publication allows statisticalmethods to be used to determine the presence and/or identity of an RFIDtag within the interrogation field to a high degree of certainty.

Turning now to FIG. 54, illustrated is a waveform diagram of anembodiment of a non-coherently integrated correlation response for anRFID tag matching a reference code or signature in accordance with aninterrogator constructed according to the principles of the presentinvention. Here two values are calculated for the correlation response,namely, a “narrow-band” signal to noise ratio (“SNR”) and a “wide-band”signal to noise ratio (“SNR”). Based upon implementation and applicationconstraints, the correlation response is partitioned into two parts,namely, data in the two end areas (designated “Narrow”) and the data inthe center (designated “Wide”). The data in the end areas of thecorrelation response are referred to as the “narrow-band” SNR values,and the data in the center of the waveform are referred to as the“wide-band” SNR values. The narrow-band SNR is the ratio of the peakvalue within the narrow-band area to the average value within thenarrow-band area, using absolute values.

SNR_(narrow)=|narrow|max/|narrow|

Similarly, the wide-band SNR is the ratio of the peak value within thewide-band area to the average value within the wide-band area, usingabsolute values.

SNR_(wide)=|wide|max/|wide|.

Turning now to FIG. 55, illustrated are waveform diagrams of anembodiment of a non-coherently integrated correlation response for anRFID tag that matches a reference code or signature (waveform designated5505), for an RFID tag with a similar reply code to the reference code(waveform designated 5510), and for no RFID tag present (waveformdesignated 5515) in accordance with an interrogator constructedaccording to the principles of the present invention. Note that on thewaveforms a significant portion of the correlation data (the“correlation triangle”) is split between the two ends of the waveform,while the information of least interest occupies the center of thewaveform. Thus, on the good correlation results, the waveform 5505 forthe RFID tag shows significant results of the correlation at ends(designated 5520) thereof, and the least significant information is atthe center (designated 5530) thereof.

The waveform 5510 for the RFID tag with a similar reply code to thereference code demonstrates about a three decibel difference from theRFID tag in the waveform 5505. Note that the magnitude of the peaks atthe ends of the waveform is significantly smaller than that in thewaveform 5505. Not as obvious is the fact that the average of the peaksin the center portion of the waveform is higher than the same averagefor the waveform 5505. The waveform 5515, where no RFID tag is present,provides a more extreme case.

Comparison of narrow-band SNR values to wide-band SNR values provides anindication of the quality of the match, with greater difference betweenthe narrow-band SNR and wide-band SNR indicating a stronger match to thereference waveform. This information is then suitable for use withstatistical analysis methods using multiple samples to determine thepresence and/or identity of an RFID tag.

Thus, a control and processing system or subsystem for an interrogator,an interrogation system, and a method of verifying a presence of an RFIDobject has been introduced herein. In one aspect, the control andprocessing system includes a memory configured to store a reference code(e.g., a synthetically derived reference code). In one aspect, thecontrol and processing system includes a correlation subsystemconfigured to correlate the reference code with a reply code from anRFID object and provide a correlation signal therefrom. The correlationsignal includes a narrow-band area having a narrow-band SNR value and awide-band area having a wide-band SNR value. The control and processingsystem also includes a decision subsystem configured to verify apresence of the RFID object as a function of the correlation signalwherein a probability of a match between the reference code and thereply code increases with a greater difference between the narrow-bandSNR value and the wide-band SNR value. Regarding an operation of thecontrol and processing system, the reference code may be provided duringan initialization stage of operation and the reply code may be providedduring a post-initialization stage of operation.

In a related aspect, the narrow-band SNR value is a ratio of an absolutevalue of a peak value within the narrow-band area to an absolute valueof an average value within the narrow-band area. The wide-band SNR valueis a ratio of an absolute value of a peak value within the wide-bandarea to an absolute value of an average value within the wide-band area.Also, the narrow-band area is typically located proximate an end of thecorrelation signal and the wide-band area is typically located proximatea center of the correlation signal.

In one aspect, the correlation subsystem includes multiple correlatorsand the correlation subsystem is configured to correlate in a timedomain employing an exclusive OR function or correlate employing a FastFourier Transform and a convolution theorem. In another aspect, thecorrelation subsystem includes a correlator configured to correlate atleast two bits of the reference code with at least two bits of the replycode to derive correlation triangles. A correlation threshold sense ofthe correlation subsystem is configured to compare the correlationtriangles to a threshold criteria to derive pulses to ascertain peaks ofthe correlation triangles. A summer of the correlation subsystem isconfigured to average a plurality of pulses from the correlationthreshold sense to provide the correlation signals.

In another aspect, the decision subsystem includes a threshold detectorconfigured to compare the correlation signal to a threshold. Thedecision subsystem may also be configured to verify the presence of theRFID object by employing a statistical analysis on a result therefrom.

A method is proposed to improve the RFID tag detection capabilities ofthe non-coherent integration correlation system described in the '450Publication under conditions of signal attenuation or radio frequencynoise and interference so severe that a single or small number ofinterrogations will not produce the level of accuracy required todeclare an RFID tag present or absent within the interrogation field. Asmentioned above, passive and semi-active RFID tags operate on theprinciple of an interrogator sending out interrogating signals bymodulating an RF carrier and then receiving a response from an RFID tagby having that RFID tag modulate its backscatter characteristics in acontrolled manner. In so doing, a unique modulated response is sent backto the interrogator or reader wherein it is detected and decoded. Due totheir small size and low cost, systems like this are desirable in manyapplications including supply chain management, inventory control, andthe general counting of and/or accounting for items in variousindustries and market segments.

Recent advancements in RFID interrogation architecture such as disclosedin the '450 Publication have greatly improved the ability of asensitivity of the interrogator for such a passive or semi-activesystem's ability to be able to accurately and reliably receive anddecode a very weak Tag-to-Reader signal. The attenuation and radiofrequency noise/interference in some environments may be so severe thatthe results of a single correlation, or a small number of correlations,are inadequate to declare, with a high degree of certainty, the presenceor absence of any RFID tag, or of a specific RFID tag. What is needed,therefore, is a method to process the results of many probabilisticresults and obtain an indication of the presence or absence of an RFIDtag within the interrogator's field, or to identify a specific RFID tag,with a defined degree of certainty.

In one aspect, an interrogator constructed according to the principlesof the present invention can accumulate the results for samples thatshow an RFID tag “possibly present,” and test the accumulated valueagainst an empirically derived threshold. In order to prevent unboundedaccumulation of “possibly present” samples, the accumulation methoddecrements or decays the accumulated value based upon the raw number ofsamples examined. It is only when the accumulated value exceeds aconstant that the RFID tag is declared present. The decay constant anddetection threshold constant may be selected to trade accuracy againstsensitivity. Preferably, a minimum number of samples are employed beforea result can be declared.

Turning to FIGS. 56 and 57, illustrated are flow diagrams of embodimentsof methods of operating an interrogation system according to theprinciples of the present invention. Here, a waveform sample is obtainedand analyzed using a method that produces a statistically significant,but not definitive indication of RFID tag presence, expressed as aresult dimensionless number in a step 5605. This number is then used asan input to a process as set forth below. The method first “decays” ordecrements the existing value of the result accumulator by an amountknown as the “decay constant” in a step 5610, then adds the resultnumber to the result accumulator in a step 5615. The new value of theresult accumulator is tested to see if it exceeds the constant“detection threshold” in a step 5620. If it does, detection is reportedin a step 5630, otherwise, no detection is reported in a step 5625.

In another embodiment illustrated with respect to FIG. 57, the methoddescribed in FIG. 56 is modified to ensure that a minimum number ofsamples are considered prior to declaring detection. A “sample counter”indicating the number of samples included in the accumulated result isincremented in a step 5705. When the result accumulator exceeds thedetection threshold, an additional test is performed to ensure theminimum number of samples has been reached in a step 5710 prior todeclaring detection.

Additionally, combining the concepts discussed above can create systemswith greatly enhanced overall detection capabilities and withcapabilities greater than any of the above concepts when consideredindividually. This enhanced capability is referred to as deep scan. Asan example, combining the teachings of the '450 Publication with, forinstance, the power management and partial RFID tag response detectioncan provide a deep scan system. Of course, other concepts may also becombined with the deep scan system to further enhance detectionsensitivity.

The present invention is directed, in general, to communication systemsand, more specifically, to an identification system for a metalinstrument and an interrogation system employing the same. In accordancetherewith, the present invention provides a metal instrument includingan RFID chip and a coupler configured to couple the RFID chip to themetal instrument to allow the metal instrument to serve as an antennatherefor. The metal instrument may also include an insulator and strap,and an inductive loop. The metal instrument may also include adepression for the RFID chip and coupler, and protected by anencapsulant. The metal instrument may be a medical instrument and theRFID chip may include information associated with the metal instrument.

Turning now to FIGS. 58 and 59, illustrated are diagrams of embodimentsof an RFID tag according to the principles of the present invention. TheRFID tag includes an outer covering or encapsulant 5805 that protectsthe tag from the environment. Also, the RFID tag includes an antenna5810 that receives signals from an interrogator, and in the case of apassive tag responds to the interrogator by matching and mismatching thesame antenna. Central to the RFID tag is an assembly that contains theactive semiconductor element. Regarding FIG. 59, the assembly is shownin greater detail including a supportive element (often called a strap)5920 upon which an RFID chip 5930 is placed. Conductive elements 5925perform the electrical connection. This is but one example of an RFIDtag, and other architectures may be employed as well.

Turning now to FIG. 60, illustrated are pictorial representations ofmetal instruments (e.g., medical instruments) employable with theinterrogation system of the present invention. It can be seen that themedical instruments are of various sizes and shapes, but in this case,made principally of metal.

Turning now to FIGS. 61 and 62, illustrated are top and side views of anembodiment of a metal instrument including an RFID tag according to theprinciples of the present invention. The RFID tag includes an RFID chipmounted on a strap and placed on an insulator on to which are placed (byprinting or other means) inductive loops, all of which are attached to ametal foundation of the metal instrument. The inductive loops mayfunction as near field elements. The inductive loops may act aselectromagnetic launching elements for launching energy into and fromthe metal foundation of the metal instrument so that the metalfoundation itself becomes the antenna or a portion thereof. In thoseinstances wherein the metal foundation is used as at least a portion ofthe antenna, a coupler, acting as an impedance transformer, may benecessary to more efficiently get the RFID energy into and out of theRFID chip. The subassembly including the coupler may also be bonded ontothe surface of the metal foundation. A depression 6115 within the metalfoundation may also be provided whereby the subassembly is placed andthen covered with a suitable packaging such as a dielectric materialand/or encapsulant for protection.

Turing now to FIG. 63, illustrated is a side view of an embodiment of ametal instrument including an RFID tag according to the principles ofthe present invention. The RFID tag includes an RFID chip 6305 and astrap 6310 along with an antenna 6315 mounted within a cavity 6320hollowed out from a long portion of a metal foundation 6325 of the metalinstrument. The RFID tag may be suspended within the cavity 6320 by aninsulator or dielectric material 6330 so as to form a sufficientdistance from the metal foundation 6325 of the metal instrument so as toallow proper electromagnetic field launch. The cavity 6320 is thenfilled with dielectric material or encapsulant (a portion of which isdesignated 6335) to protect the RFID tag and may also assist inproviding a proper impedance match. In this embodiment, the cavity 6320functions as a reflector of the radio frequency energy similar in mannerto a reflector antenna as little or no physical connection existsbetween the antenna 6315 and the metal foundation 6325.

Thus, a metal instrument for use with an interrogator and aninterrogation system has been introduced herein. In one aspect, themetal instrument (e.g., a medical instrument) includes a metalfoundation, an RFID chip, and a coupler configured to couple the RFIDchip to the metal foundation to allow at least a portion of the metalfoundation to serve as an antenna thereby forming at least a portion ofan RFID tag with the RFID chip. The coupler may be bonded to a surfaceof the metal foundation.

In other aspects, the metal instrument includes an insulator locatedbetween the coupler and the metal foundation, and a strap locatedbetween the RFID chip and the coupler. Additionally, the metalinstrument also includes an inductive loop about the RFID chip locatedon the insulator coupled to the RFID chip. The metal instrument alsoincludes a depression in the metal foundation for the RFID chip and thecoupler. The metal instrument also includes an encapsulant configured toencapsulate the RFID chip and the coupler. Additionally, the RFID chipincludes information associated with the metal instrument.

In another aspect, an interrogation system includes a metal instrumentincluding a metal foundation, an RFID chip, and a coupler configured tocouple the RFID chip to the metal foundation to allow at least a portionof the metal foundation to serve as an antenna thereby forming at leasta portion of an RFID tag with the RFID chip. The interrogation systemalso includes a sensing subsystem configured to provide a signal havingat least one of a metal signature representing a presence of the metalfoundation and an RFID signature representing a presence of the RFIDtag. The interrogation system also includes and a control and processingsubsystem configured to process the signal to discern a presence of atleast one of the metal foundation and the RFID tag.

In another aspect, the metal instrument (e.g., a medical instrument)includes a metal foundation with a cavity (e.g., hollowed from a longportion of the metal foundation), an insulator within the cavity, and anRFID tag separated from the metal foundation within the cavity by theinsulator. The cavity is configured to act as a reflector of radiofrequency energy associated with the RFID tag.

In other aspects, the RFID tag includes an RFID chip and a strap. Also,the cavity is filled with an encapsulant to protect the RFID tag or toenhance an impedance match for the RFID tag. The RFID tag is separatedfrom the metal foundation within the cavity by a distance to allow anelectromagnetic field launch. Additionally, the RFID chip includesinformation associated with the metal instrument.

In another aspect, an interrogation system includes a metal instrumentincluding a metal foundation with a cavity, an insulator within thecavity, and an RFID tag separated from the metal foundation within thecavity by the insulator. The cavity is configured to act as a reflectorof radio frequency energy associated with the RFID tag. Theinterrogation system also includes a sensing subsystem configured toprovide a signal having at least one of a metal signature representing apresence of the metal foundation and an RFID signature representing apresence of the RFID tag. The interrogation system also includes acontrol and processing subsystem configured to process the signal todiscern a presence of at least one of the metal foundation and the RFIDtag.

Considering surgical procedures, studies have shown that approximately76 percent of all items unintentionally retained within a patient as theresult of a procedure are categorized as surgical sponges. The surgicalsponges are all too often left within the patient, even though themedical staff exercises extraordinary procedures to prevent this.Medical emergencies and time pressures provide a fertile environment forwhat is still a manual procedure.

Turning now to FIG. 64, illustrated is a pictorial representation of anexemplary counting system for surgical sponges. The sponge countersystem, essentially a plastic sheet with pockets and suspended on apole, is used to separate and count sponges (one of which is designated“SPONGE”). These manual systems require active control of theindividual(s) responsible for counting, and are prone to error as bloodand fluid soaked sponges can easily be lumped together and thereforeappear as a single sponge so that multiple sponges can erroneously beplaced into a single pocket. Finding such an erroneously placed spongeand correcting the action can take considerable time as it is not knownwhether or not the sponge may still be retained within the patient.Additionally, even when found, the risk to the patient has beenincreased as the time when the surgical opening can be closed must bedelayed until the count is correct. Not finding a sponge that hasinadvertently been left in a patient during a surgical procedure issufficiently common to have been given the clinical name of gossipyboma.Multiple cases exist within the literature of gossipyboma causing majorsuffering, irreparable damage to the health of the patient, and evenincluding death.

Therefore, what is needed is an accurate enabling system that providesthe means for surgical sponges to be automatically detectable prior tothe procedure, during the procedure when they are intracorporeal, andpost procedure when they are either soiled, or have never been used sothat full accountability is always accurately maintained. In accordancetherewith, a sponge according to the present invention includes an RFIDtag, special encapsulation for the RFID tag, and a means to affix theRFID tag to the sponge. Additionally, the sponge may include aradiopaque object, special encapsulation for the object, and a means toaffix the object to the sponge. The sponge may also include a designator(e.g., surface designation) as set forth herein.

A tagged sponge system is described that includes at least one surgicalsponge, one encapsulated RFID tag, and the means to permanently affixthe RFID tag to the sponge. The RFID tag provides the means by which thesponge is at all times detectable when used in conjunction withinterrogators.

Turning now to FIGS. 65 to 68, illustrated are pictorial representationsof several types of surgical sponges. These examples are not meant to beexhaustive, but only serve to show some of the various types and shapesconsistent with a surgical sponge. The sponge of FIG. 65 is commonlyreferred to as a LAP sponge or a 4×4 including several layers of washedcotton sewn together and typically has a loop 6510 sewn into it. Thisloop 6510 is permeated with a radiopaque material, typically bariumsulphate. The purpose of this radiopaque loop 6510 is so that a spongeinadvertently left within a patient can be discovered through an x-rayprocedure. Should an x-ray procedure be necessary to find an errantsponge, the time to accomplish this is in the range of 15 to 45 minutesAs operating room time is in the range of $45 to $90 per minute, theexpense of requiring such an operation can be considerable.Additionally, these impregnated loops 6510 are typically judged to beonly about 80 percent effective as a detection mechanism, so even thoughgreat effort and expense is incurred, the results can still beunacceptable and even disastrous.

Turning now to FIG. 66, illustrated is another example of a surgicalsponge, often referred to as a RAY-TEC. This sponge is approximately thesame size as the LAP sponge (four inches square), but contains lesscotton material. Additionally, radiopacity is accomplished here viaimpregnated threads 6620. Given that loop 6510 is only about 80 percenteffective, the much smaller threads 6620 are therefore deemed to be lessthan 80 percent effective, once again indicating a significant problem.

Turning now to FIG. 67, illustrated are examples of what are oftencalled tonsil sponges 6730. These are spherical in shape and can be usedto separate muscle tissues or organs and are typically 0.5 inches indiameter. When used in this manner they can be obstructed (i.e., hiddenfrom view) by that same muscle tissue or organ. Radiopacity is achievedhere by colored threads 6740, which once again are relatively sparse.

Turning now to FIG. 68, illustrated is another example of a sphericalsponge 6850 referred to as a cherry dissector with radiopacity threads6860 that are typically used in manners similar to the tonsil sponges.All these FIGUREs illustrate the need for detectability if left withinthe patient by the presence of radiopaque elements, but also show theinadequacy of current systems.

Turning now to FIG. 69, illustrated are pictorial representations ofRFID tags. These particular tags are UHF EPC Gen I and Gen II passivetags, but they are presented only as examples and other types of RFIDtags may be employed as well. Both far field RFID tags 6905, 6910 andnear field RFID tags 6920, 6930 are shown along with designs thatencompass both near and far field RFID tag characteristics 6915, 6925.Specifically referring to RFID tag 6930, the structure includes asilicon chip 6935 connected to an antenna 6940. The chip 6935 andantenna 6940 along with a structure for holding same, often referred toas the inlay is what typically constitutes a passive RFID tag. A U.S.quarter 6945 is included as a reference for the size of these RFID tags.

Turning now to FIGS. 70 to 77, illustrated are diagrams of embodimentsof a sponge in accordance with the principles of the present invention.A sponge is a reusable or consumable (or disposable) item that includesat least one layer of material and may be an absorbent surgical sponge.Beginning with FIG. 70, an encapsulant 7010 is added to protect an RFIDtag 7005 from bodily fluids and other fluids used during a surgicalprocedure. The encapsulants used would typically comply with UnitedStates Pharmacopeia (“USP”) Class IV, V, or VI standards or thestandards as specified in ISO 10993 Biological Evaluation of MedicalDevices to assure safety and biologic compatibility when within apatient. Many sources of such material exist and examples of companiesmanufacturing such materials are Master Bond, Dymax Corp., Loctite(Henkel Technologies), and Fisher-Moore. The RFID tag 7005 is completelyencapsulated by the above referenced material. In some embodiments,encapsulant thickness may vary beyond that simply employed for RFID tagprotection. Based on the type of RFID tag employed, near field or farfield, the added thickness may be employed to launch the proper fieldmode.

Turning now to FIG. 71, illustrated is a diagram of a LAP sponge 7100with an encapsulated RFID tag 7105. In this embodiment, attachmentoccurs by sewing (designated by a seam 7115) the RFID tag 7105 onto thesponge 7100 in the same area as a radiopaque object (e.g., radiopaqueloop) 7120 is attached. The sewing operation is performed on theencapsulant portion of the RFID tag 7105 thereby avoiding the actualantenna and chip areas. As illustrated, the sponge 7100 is formed frommultiple layers of absorbent material and the RFID tag 7105 is locatedwithin an interior space of the sponge 7100. Additionally, FIG. 72 is anillustration of a RAY-TEC sponge 7225 with an RFID tag 7240 attachedthereto. In this embodiment, attachment occurs by sewing (designated bya seam 7235) the RFID tag 7240 directly onto the sponge 7225 and in aregion different from radiopaque threads 7230. As illustrated, thesponge 7225 is formed from multiple layers of absorbent material and theRFID tag 7240 is located within an interior space of the sponge 7225.Additionally, ends of the ends of the layers are bound and the interiorspace is formed therebetween. In both embodiments above, the RFID tagwas sewn, but this invention comprehends embodiments where the RFID tagis affixed in any manner including bonded by an adhesive agent as well.

As mentioned above, since these sponges consist of multiple layers, theRFID tags are embedded among and within an interior space of the layersand may be affixed to an external layer of the sponge. Additionally,this invention comprehends an embodiment where the surface of theencapsulant consists of a specified roughness or texture so that itadditionally adheres to the sponge by attaching to individual fibers ina manner similar to Velcro. Additionally, this invention comprehendsembodiments wherein the RFID tag is embedded within, or attacheddirectly to, for instance, the radiopaque loop 7120 of FIG. 71 andthereby attached to the sponge at the same time as the loop 7120 is soaffixed.

Turning now to FIG. 73, illustrated is an embodiment of an RFID taggedLAP sponge 7305 as described above in FIG. 71 with an encapsulated metaltag 7310 attached to the sponge 7305 by sewing (designated by a seam7315). The purpose of the metal tag 7310 is to enhance the radiopacityof the LAP sponge 7305 forming a radiopaque element in addition to aradiopaque loop 7320. Additionally, FIG. 74 is an illustration of anembodiment of an RFID tagged RAY-TEC sponge 7425 as described above inFIG. 72 with an encapsulated metal tag 7435 attached to the sponge 7425by sewing (designated by a seam 7440) also to enhance the radiopacity inaddition to radiopaque threads 7430. In these embodiments, attachmentoccurs by sewing the metal tag directly onto the sponges and in a regionproximately close to the areas where the RFID tags have been attached.

Additionally, this invention comprehends embodiments where the metal tagis not purely metal, but a metallic compound or some other radiopaquecompound. Additionally, this invention comprehends embodiments whereinthe metal tag is bonded by an adhesive agent as well. Additionally,since these sponges consist of multiple layers, this inventioncomprehends an embodiment wherein the metal tag is embedded among andwithin an interior space of the layers and may be sewn to an externallayer of the sponge. Additionally, this invention comprehends anembodiment where the surface of the encapsulant consists of a specifiedroughness or texture so that it additionally adheres to the sponge byattaching to individual fibers in a manner similar to Velcro.Additionally, this invention comprehends embodiments where the RFID tagand the metal tag are encapsulated together into a single module.Additionally, this invention comprehends embodiments where the metal tagis embedded within, or attached to, a radiopaque loop 7120 of FIG. 71and thereby attached to the sponge at the same time as the radiopaqueloop 7120 is so affixed.

The sponges may also include a designator 7450 as indicated on theRAY-TEC sponge 7425 of FIG. 74. The designator 7450 may provide asurface designation by use of a highly visible indicator with a color,shape or pattern that is non standard (not currently used in thesurgical or medical environment and clearly visible and readilyidentifiable) and, from a distance, clearly distinguishes the sponge7425 as unique and appropriate. A unique surface color covering asignificant percentage of the surface area of the sponge 7425 may beused (die, colored threat woven into the product, a sewn band of aunique color). A unique color preferably with a luminance having a highcontrast ratio (e.g., greater than 10:1) may be beneficial. Also,preferably hydrophobic or liquiphobic colorant or material thatmaintains its color and differentiation under the majority of conditionsexpected in a healthcare setting would be beneficial. It should also beunderstood that an encapsulated RFID or metal tag may also be designatedwith a unique color, pattern or properties described above. Thedesignator 7450 may also have other designations such as a bar code. Thedesignator 7450 may also provide an indication beyond an RFID tag ormetal tag that the sponge includes such an RFID tag or metal tag.

Turning now to FIG. 75, illustrated is a front view of an embodiment ofa tagged tonsil sponge or cherry dissector sponge 7505 including anencapsulated RFID tag 7510 as described above. Turning now to FIG. 76,illustrated is a front view of an embodiment of a tagged tonsil spongeor cherry dissector sponge 7620 including an encapsulated RFID tag 7625and an encapsulated radiopaque element 7630 as described above. Turningnow to FIG. 77, illustrated is a front view of an embodiment of a taggedtonsil sponge or cherry dissector sponge 7740 including an encapsulatedRFID tag 7745 and an encapsulated radiopaque element 7750 surroundingthe RFID tag 7745. This invention comprehends embodiments of FIGS. 76and 77 wherein the RFID tag and radiopaque element consist of a singlemodule.

Thus, a sponge for use with an interrogator and an interrogation systemhas been introduced herein. In one aspect, the sponge (e.g., a LAPsponge or a RAY-TEC sponge) includes first and second layers ofabsorbent material that forms an interior space, and an RFID tag affixed(e.g., sewn or bonded by an adhesive agent) to the absorbent materialwithin the interior space.

In other aspects, the sponge also includes a radiopaque object affixedto at least one of the first and second layers of the absorbent materialand the RFID tag is affixed to the radiopaque object. Additionally, endsof the first and second layers are bound and the interior space isformed therebetween. The RFID tag includes an RFID chip and an antenna.The RFID tag is surrounded by an encapsulant and the RFID tag is affixedto the absorbent material through the encapsulant. The sponge alsoincludes an encapsulated metal tag affixed through an encapsulantthereof to at least one of the first and second layers of the absorbentmaterial. The metal tag may also be encapsulated with the RFID tag in amodule affixed to at least one of the first and second layers of theabsorbent material. The sponge may also include a designator affixed toat least one of the first and second layers of the absorbent materialthat is visible from outside of the interior space. The designator maybe a liquiphobic designator with a luminance having a high contrastratio.

In another aspect, the sponge includes at least one layer of material,an RFID tag affixed (e.g., sewn or bonded by an adhesive agent) to thematerial, and a designator (e.g., affixed to the material) that providesan indication beyond the RFID tag that the sponge includes the RFID tag.The designator may be a liquiphobic designator with a luminance having ahigh contrast ratio.

In other aspects. the sponge includes a radiopaque object affixed to thematerial and the RFID tag is affixed to the radiopaque object. The RFIDtag is surrounded by an encapsulant and the RFID tag is affixed to thematerial through the encapsulant. The sponge include an encapsulatedmetal tag affixed through an encapsulant thereof to the material. Themetal tag may also be encapsulated with the RFID tag in a module affixedto the material.

In another aspect, an interrogation system includes a sponge includingat least one layer of material, an RFID tag affixed to the material, ametal tag affixed to the material, and a designator that provides anindication beyond the RFID tag that the sponge includes the RFID tag.The interrogation system also includes a sensing subsystem configured toprovide a signal having at least one of a metal signature representing apresence of the metal tag and an RFID signature representing a presenceof the RFID tag. The interrogation system also includes a control andprocessing subsystem configured to process the signal to discern apresence of at least one of the metal tag and the RFID tag.

The present invention pertains to methods and apparatus for an improvedmetal detection assembly or metal interrogator including metal sensingsystems and subsystems and the associated control and processing systemsand subsystems. The design goals of devices of this class of equipmentas shown in FIG. 78 may include not only high sensitivity, but alsodiscrimination ability; for example, maximizing sensitivity to certainclasses of target metals, while maximizing rejection of certain otherclasses or “background metal objects” as shown in FIG. 79.

The metal interrogator is a refinement in what is known in the art as apulse induction metal detection. An exemplary embodiment of the metalinterrogator includes, among other things, an antenna array (alsoreferred to as a coil assembly or coil(s), and antenna(s)), a metalsensing subsystem (including a transmit pulse generator with a pulsecontroller, timing generator and a power driver, and receiver) and acontrol and processing subsystem (including a digital signal processor“DSP”). The antenna array may be a single coil, or multiple coils with asystem of relays connecting a selected coil to the driver and receiver.As described herein, an antenna array or coil assembly (designated “CoilAssy”) in FIG. 80 is also referred to as a coil for the sake of brevity.FIGS. 82 to 85 illustrate examples of single and multiple antenna arrayconfigurations. The antenna arrays of FIGS. 82 to 84 illustrate the useof separate transmit and receive coils. As provided herein, referencewill be made to embodiments that use an N-channel metal-oxidesemiconductor field-effect transistor (“MOSFET”) as the active elementin the power driver. Of course, different types of active devices mayalso be used if the polarity of power supplies and if thedirection/polarity of diodes are reversed, or multiple transistors maybe used in parallel, or specific pulse shaping may also be desirable.

Turning now to FIGS. 80 and 81, illustrated are block diagrams of anembodiment of a metal interrogator constructed according to theprinciples of the present invention. The metal interrogator includes acontrol and processing subsystem (e.g., a digital signal processor(“DSP”)) that through application of a timing generator of a transmitpulse generator feeds a control pulse of selected length to a gateterminal of a switch such as a MOSFET (e.g., an N-channel MOSFET) of thepower driver causing the MOSFET, acting as a switching element, to turnON for a short length of time. A direct current (“DC”) power supply ofthe power driver with high short-term pulse current capability providespulse energy to be fed into the magnetic field in the vicinity of thecoil assembly. Its negative terminal is connected to a source terminalof the N-channel MOSFET, and its positive terminal is connected to oneterminal of the coil assembly. The other terminal of the coil assemblygoes to a drain terminal of the N-channel MOSFET, either directly orthrough a divorcing diode (designated “Diode”) of the power driver to bedescribed shortly. The energizing circuit within this embodiment is aloop of three elements including the power supply, coil assembly, andN-channel MOSFET, or correspondingly, a loop of four elements if thedivorcing diode is in use, between the drain of the MOSFET and the coilassembly.

In this embodiment, the non-switched terminal of the coil assembly isreferenced to ground potential, thereby making the power supply for thecoil assembly a negative voltage supply, and also determines the form ofpulse for the gate of the N-channel MOSFET. Of course, the coil assemblymay not be specifically grounded. The potential for the power supply ofthe coil assembly may be, for example, −12 volts or −24 volts. A peakcurrent reached by the end of the energizing pulse may reach, forexample, 5, 10, 30 or 40 amps, and may be programmed through the choiceof the pulse length and coil inductance. Exemplary pulse lengths arewithin the range of 5-40 microseconds.

The divorcing diode (again, designated “Diode”) detects the effects of aresponse (e.g., decaying eddy currents) from the target metal objectvery early after the MOSFET switch turns OFF. The diode is selectedusing criteria of very fast reverse recovery time, high pulse currentcarrying capability, and low reverse capacitance. The cathode of thediode goes to the drain terminal of the N-channel MOSFET, and the anodegoes to the coil assembly. While the MOSFET is turned ON, the diode isforward biased, and the large current building in the coil assemblypasses therethrough. When the MOSFET switch turns OFF, the originalcurrent path through the MOSFET's conducting channel region becomesblocked, but because of the energy now stored in the magnetic fieldaround the coil assembly, the current in the coil keeps flowing. Atfirst, the current still flows mostly through the MOSFET. Even thoughthe MOSFET is now an open switch, it has a parasitic capacitance on theorder of hundreds of picofarads. The current goes to charge thiscapacitance, and as a result the drain voltage rises, perhaps to around+400 volts. Typically, a high voltage MOSFET is chosen to allow this,because higher voltages mean the energy comes out of the coil's magneticfield faster. The voltage may rise to the point where the MOSFET'sinternal limiting breakdown diode limits the energy, absorbing much ofthe coil's stored energy or, in another embodiment, an optionalauxiliary circuit (not shown) may be added to limit the peak and absorbsome of the energy, possibly even for re-use.

After a time, perhaps on the order of a microsecond, the energy in thecoil's magnetic field and its terminal current reaches zero. This is animportant transition toward the goal of making the applied pulse energysubstantially disappear so that the tiny amounts of energy in the formof a response from the target metal object can be detected. However, theenergy decay is not finished yet at this point, because now there arestill substantial (on the order of 400 volts) parasitic and straycapacitances associated with the coil's switched terminal, and thismeans there is still stored energy. The circuit now takes on thecharacteristic of a capacitor-inductor resonator, capable of storingenergy in resonant form, oscillating between magnetic energy in the coilassembly and electric field energy in the parasitic capacitance. Acommon solution is to have a load resistor placed across the coilterminals, chosen according to textbook formulas to approximate what iscalled “critical damping.” An active circuit may be employed in place ofthe load resistor and actively control the speed at which energydissipates.

An advantage of using the divorcing diode approach is that when thezero-current transition occurs, the diode becomes reverse-biased andtherefore quickly becomes non-conductive. Instead of having on the orderof 1000 picofarads of MOSFET drain capacitance connected to the coilassembly, the diode only presents on the order of 20 picofarads. Theselection of the load resistor for the goal of critical damping shouldtake this smaller value into account; and the result is a fasterexponential decay of the residual energy. Furthermore, at the instantthe diode begins to conduct, most of the residual energy in the systembecomes isolated within the parasitic capacitance of the MOSFET, whereinit is trapped until the end of the detection cycle. The receiverincludes a protective clamping circuit followed by an amplifier and,optionally, a logarithmic converter. The results are fed to ananalog-to-digital converter (“ADC”) and converted to sampled numericalwaveforms for processing.

Turning now to FIG. 90, illustrated is a functional block diagram ofportions of an embodiment of a metal interrogator constructed accordingto the principles of the present invention. An antenna array 9000includes individual metal detection antennas (designated “Ant”) coupledto a metal sensing subsystem 9005. As illustrated in FIG. 80, the metalsensing subsystem 9005 includes, in an exemplary embodiment, a transmitpulse generator and a receiver. Each of these antennas emit anelectromagnetic pulse, then accept the decay characteristics of aresponse caused by the environment. The response is digitized within themetal sensing subsystem 9005, also referred to as a sample. Single ormultiple samples may be taken from each antenna or a group of antennas.Two or more samples from a single antenna may be processed to reducenoise in a stage known as multi-sample noise reduction subsystems 9010.The resultant digital waveform from the multi-sample noise reductionsubsystems 9010 may have further noise removal done by intra-samplenoise reduction subsystems 9015. Other embodiments may reverse the orderof these noise reduction subsystems, or perform only one stage or theother or no noise reduction. Then, a detection subsystem 9020 analyzesthe resultant waveform or waveforms and produces a detection result9025. In the illustrated embodiment, a digital signal processorincluding the multi-sample noise reduction subsystems 9010, theintra-sample noise reduction subsystems 9015 and the detection subsystem9025 forms a control and processing subsystem for the metalinterrogator.

Turning now to FIG. 91, illustrated are digitized waveform diagramsdemonstrating waveforms produced by a metal interrogator in accordancewith the principles of the present invention. Detection of a metalobject within the area being scanned is accomplished by analysis ofthese waveforms. A waveform 9100 is illustrated from a single sampledresponse. Note the deviations from the smooth curve of the waveform,particularly in the latter (rightmost) parts of the sample. Theseirregularities are produced by environmental and uncorrected equipmentnoise and are collectively referred to as noise. The first stage of anymetal detection process is the reduction of sample noise. Two generalclasses of noise reduction are used, namely, multi-sample noisereduction and intra-sample noise reduction.

Multi-sample noise reduction includes taking multiple samples from asingle antenna within close temporal bounds. Since much of the noiseseen in the waveform 9100 is random, noise should occur at differenttimes within different samples, allowing multiple samples to be analyzedon a point by point basis to determine the invariant part of the sample.Illustrated in waveform 9101 is the result of deriving a sampledresponse with reduced noise from analysis of multiple sampled responsesfrom the same antenna.

Turning now to FIG. 92, illustrated are exemplary waveforms diagrams9200, 9201, 9202 of three sampled responses taken from the same antennawithin a short time interval in accordance with the principles of thepresent invention. Note that waveform 9201 of the second sampledresponse has a noise spike 9203. Multi-sample noise reduction produces awaveform 9204 of the resultant single sampled response. Note thatdepending upon the noise reduction process used, the noise spike 9203may be eliminated, or merely reduced as illustrated by noise spike 9205in the waveform 9204 of the resultant single sampled response. Methodsused to analyze multiple sampled responses from the same antenna toreduce noise include, but are not limited to, simple averaging, medianfiltering, and also performing an initial sort, and then discardingoutliers via preprogrammed decisions. For example, this can be doneassuming uniform, Gaussian, Cauchy, or other distributions. Theselection of filtering method is driven by a need to reduce the numberof samples to achieve a given reduction in noise. A person skilled inthe art can derive other methods of noise reduction using multipleredundant sampled responses.

Turning now to FIG. 93, illustrated are waveform diagrams ofintra-sample noise reduction that relies upon the fact that theresultant waveform from an induction pulse is an exponential curve inaccordance with the principles of the present invention as exemplifiedin waveform 9300. Short duration random and non-random noise appears tobe a spike 9301 on this otherwise smooth curve. Noise reduction methodsthat take advantage of the smoothness of the curve may be used here.These methods involve examining a “window” 9302 or range of adjacentdata points on the curve (usually a small, odd number such as three orfive) centered on a specific point X. These processes generate a new,reduced noise curve by setting point X′ in the new curve to be a“reasonable” value given examination of the points adjacent to X. Thus,if a noise spike occurred at point X, the points immediately before andafter X will probably be non-noise points, and the value of X can beinferred from these points. Typically, this is done by setting X to theaverage or median of the values of all points in the examination window9302, but other methods exist and are obvious to one skilled in the art.A side-effect of this type of analysis is a shifting or delaying of theresultant sample, but this may be accounted for in the analysis system.Choice of window size is determined by anticipated noise spike duration(tends to drive the filter window wider) versus processing time (tendsto drive the window narrower).

Detection systems take N noise-reduced sampled responses, where N is thenumber of distinct metal detection antennas, and produce a result thatindicates the probability of a metal object of interest being near aparticular antenna or antennas. This result may be combined with priorresults via various methods as a further type of noise reduction. Threedetection methods known as the residual method, the slope-differentialmethod, and the curve crossing detection method may be employed withoutlimitation.

Turning now to FIGS. 94 and 95, illustrated are flow diagrams ofembodiments of a residual method metal detection process for a metalinterrogator in accordance with the principles of the present invention.The process of FIG. 94 is illustrated without using a reference and theprocess of FIG. 95 is illustrated using a reference. The discussion thatfollows describes the metal detection process in accordance withoutusing a reference as illustrated in FIG. 94. In a step 9400, sampledresponses are collected via an antenna array and metal sensing subsystemand, in a step 9401, noise reduction is performed on the sampledresponses as discussed previously. In a step 9402, a control andprocessing system of the metal interrogator computes a “background”ambient signal level for each antenna by averaging the sampled responsesfor all antennas, preferably without the current antenna. Thisbackground ambient signal level is consistent on a pulse by pulseduration. The changes in this signal are typically larger than thevariation in the response due to the presence the target metal object.As a result, it may be advantageous to estimate the background ambientsignal level on a pulse by pulse basis. This background ambient signallevel is then subtracted point by point from the antenna's currentsample.

Thus, the residual for antenna 0 in a four antenna array would be thedifference between the antenna's sample and the average of the samplesfrom antennas 1, 2 and 3. This effectively removes any signal common toall other antennas, thereby tending to reduce the effect of metalobjects common to all antennas such as an operating room table. In astep 9403, the control and processing subsystem determines the areaunder the curve for each antenna's residual level. Since the presence ofa metal object in an antenna's field reduces the initial portions of thecurve, the integral of the curve is lower. In steps 9404, 9405 and 9406,the control and processing subsystem normalizes the integral values sothat the lowest value is set to zero and, in a step 9407, the controland processing subsystem converts the values to a logarithmic scale tofacilitate evaluation. In a step 9408, the control and processingsubsystem detects the metal object by selecting the lowest of theintegral values.

Turning now to FIG. 96, illustrated are waveform diagrams 9600, 9601 ofsampled responses from two antennas, which are digitized and sampled,according to the principles of the present invention. Slope-differentialanalysis includes measuring the slope between two points X and Y, andcomparing the resultant slopes 9603, 9605. Since the presence of a metalobject in an antenna's scan field causes a resultant waveform with aflatter early (leftmost) portion, the presence of a metal object may beinferred from a lower slope. The choice of values of X and Y and theconsequent slope location and slope length may by static or dynamicallyaltered within the scope of the present invention.

Turning now to FIG. 97, illustrated are waveform diagrams 9700, 9701 forsampled responses from two different metal detection antennas inaccordance with the principles of the present invention. A metal objectis within the detection range of the antenna associated with the sampledresponse shown in waveform 9701, while the antenna associated withsampled response shown in the waveform 9700 has no metal object withinrange. Note the curve in waveform 9701 shows a lower amplitude than thatof waveform 9700 during the earlier (leftmost) time interval, but showsa similar amplitude at a point late Y for the waveforms 9700, 9701,respectively. The curve crossing detection method determines theamplitude of the curve at a point X. Sampled responses with a loweramplitude at point X than sampled responses from other antennas may beinferred to have a metal object within detection range. Since thesteady-state or rightmost portions of the waveforms 9700, 9701 areunaffected by metal objects, these may be used to dynamically determinethe appropriate value of X as a self-calibration method. This logic mayalso be reversed, in which the value of X for a specific Y value isdetermined, where Y may be based upon a percentage of the final orrightmost parts of the curve. The full curve-crossing detection methodmay also contain logic to compensate for non-identical behavior amongantennas and the presence of large metal objects (such as an operatingtable) within the scan field of all antennas. Additional exemplaryantenna arrays shown in FIGS. 86 to 89 illustrate some of the variousgeometric structures that may be employed to enhance detection. Themetal interrogator comprehends the use of multiple antenna arrays notshown in these FIGUREs.

Note that the detection processes described herein are illustrative andmay be enhanced or modified by one skilled in the art. Suchmodifications may include, but are not restricted to, addition ofcalibration values to counteract differences in antenna characteristics,addition of calibration values used to define empty space or the lack ofmetal objects within the scan field, addition of logic to infer metalobject location, size, orientation or composition, addition of logic todetermine spatial placement of a metal object within a larger scan area(such as a surgical patient), changes to excitation pulse width oramplitude, and permutations of antenna sequencing, sample temporalspacing and sample sequencing.

Thus, a metal interrogator for use with an interrogation system, and amethod of operating the same has been introduced herein. In one aspect,the interrogator includes an antenna array having coils (e.g., aplurality of overlapping coils) that define multiple areas, and a metalsensing subsystem configured to transmit a pulse to each coil of theantenna array and receive a response (e.g., a decaying eddy currentresponse) therefrom. The interrogator also includes a control andprocessing subsystem configured to estimate and subtract a backgroundsignal level from the response from each coil and provide a residualtherefrom, thereby discerning a presence of a metal object in at leastone of the multiple areas. The background signal level may include abackground metal object proximate the metal object.

In other aspects, the metal sensing subsystem includes a transmit pulsegenerator including a pulse controller, a timing generator and a powerdriver. The transmit pulse generator is configured to transmit thepulse. The metal sensing subsystem also includes a receiver including aclamping circuit, amplifier and analog to digital converter. Thereceiver is configured to receive the response and provide a sampledresponse to the control and processing subsystem. The control andprocessing subsystem (e.g., a digital signal processor) is configured toestimate and subtract the background signal level from the sampledresponse.

In still other aspects, the control and processing subsystem includes amulti-sample noise reduction subsystem, an intra-sample noise reductionsubsystem and a detection subsystem. The control and processingsubsystem is configured to estimate the background signal level for eachcoil by averaging the responses for the coils. The control andprocessing subsystem is configured to subtract the background signallevel point by point from the response. The control and processingsubsystem is also configured to normalize an integral value of theresidual. The control and processing subsystem is also configured tonormalize an integral value of the residual for each coil and select alowest value therefrom to discern the presence of the metal object.

Exemplary embodiments of the present invention have been illustratedwith reference to specific electronic components. Those skilled in theart are aware, however, that components may be substituted (notnecessarily with components of the same type) to create desiredconditions or accomplish desired results. For instance, multiplecomponents may be substituted for a single component and vice-versa. Theprinciples of the present invention may be applied to a wide variety ofapplications to identify and detect objects. For instance, in a medicalenvironment, instrument kits including a plurality of objects can bescanned in situ to log the contents thereof into an interrogator, andsubsequently the instrument kit can be scanned by the interrogator toverify the contents, the integrity of the contents (including expirationdates for time sensitive objects) and the like. The increasedsensitivity of the interrogator according to the principles of thepresent invention opens up many new opportunities (e.g., supply chainmanagement in consumer related retail applications, securityapplications, etc.) for the interrogation system disclosed herein.

Also, although the present invention and its advantages have beendescribed in detail, it should be understood that various changes,substitutions and alterations can be made herein without departing fromthe spirit and scope of the invention as defined by the appended claims.For example, many of the processes discussed above can be implemented indifferent methodologies and replaced by other processes, or acombination thereof, to form the devices providing reducedon-resistance, gate drive energy, and costs as described herein.

Moreover, the scope of the present application is not intended to belimited to the particular embodiments of the process, machine,manufacture, composition of matter, means, methods and steps describedin the specification. As one of ordinary skill in the art will readilyappreciate from the disclosure of the present invention, processes,machines, manufacture, compositions of matter, means, methods, or steps,presently existing or later to be developed, that perform substantiallythe same function or achieve substantially the same result as thecorresponding embodiments described herein may be utilized according tothe present invention. Accordingly, the appended claims are intended toinclude within their scope such processes, machines, manufacture,compositions of matter, means, methods, or steps.

1. A control and processing system, comprising: a correlation subsystemconfigured to correlate a reference code at a first clock rate with areply code from a radio frequency identification (RFID) object,correlate said reference code at a second clock rate with said replycode from said RFID object and provide correlation signals therefrom,said correlation subsystem being configured to correlate employing aFast Fourier Transform and a convolution theorem; and a decisionsubsystem configured to verify a presence of said RFID object as afunction of said correlation signals.
 2. The control and processingsystem as recited in claim 1 wherein said correlation subsystem includesmultiple correlators.
 3. The control and processing system as recited inclaim 1 wherein said reference code is a synthetically derived referencecode.
 4. The control and processing system as recited in claim 1 whereinsaid correlation subsystem is configured to correlate in a time domainemploying an exclusive OR function.
 5. The control and processing systemas recited in claim 1 wherein said first clock rate is an expected clockrate and said second clock rate varies therefrom.
 6. The control andprocessing system as recited in claim 1 wherein said correlationsubsystem includes: a correlator configured to correlate at least twobits of said reference code with at least two bits of said reply code toderive correlation triangles, a correlation threshold sense configuredto compare said correlation triangles to a threshold criteria to derivepulses to ascertain peaks of said correlation triangles, and a summerconfigured to average a plurality of pulses from said correlationthreshold sense to provide said correlation signals.
 7. The control andprocessing system as recited in claim 1 wherein said decision subsystemincludes a threshold detector configured to compare said correlationsignals to a threshold.
 8. The control and processing system as recitedin claim 1 wherein said decision subsystem is configured to verify saidpresence of said RFID object by employing a statistical analysis on aresult therefrom.
 9. The control and processing system as recited inclaim 1 wherein said reference code is configured to be provided duringan initialization stage of operation and said reply code is configuredto be provided during a post-initialization stage of operation.
 10. Thecontrol and processing system as recited in claim 1 wherein saidreference code is configured to be entered into a memory of said controland processing system.
 11. A method of verifying a presence of a radiofrequency identification (RFID) object, comprising: correlating areference code at a first clock rate with a reply code from an RFIDobject, correlating said reference code at a second clock rate with saidreply code from said RFID object and providing correlation signalstherefrom, said correlating employing a Fast Fourier Transform and aconvolution theorem; and verifying a presence of said RFID object as afunction of said correlation signals.
 12. The method as recited in claim11 wherein said correlating is performed with multiple correlators. 13.The method as recited in claim 11 wherein said reference code is asynthetically derived reference code.
 14. The method as recited in claim11 wherein said correlating is performed in a time domain employing anexclusive OR function.
 15. The method as recited in claim 11 whereinsaid first clock rate is an expected clock rate and said second clockrate varies therefrom.
 16. The method as recited in claim 11 whereinsaid correlating includes: correlating at least two bits of saidreference code with at least two bits of said reply code to derivecorrelation triangles, comparing said correlation triangles to athreshold criteria to derive pulses to ascertain a peak of saidcorrelation triangles, and averaging a plurality of pulses to providesaid correlation signals.
 17. The method as recited in claim 11 whereinsaid verifying includes comparing said correlation signals to athreshold.
 18. The method as recited in claim 11 wherein said verifyingemploys a statistical analysis on a result therefrom.
 19. The method asrecited in claim 11 wherein said reference code is configured to beprovided during an initialization stage of operation and said reply codeis configured to be provided during a post-initialization stage ofoperation.
 20. The method as recited in claim 11 further comprisingentering said reference codes into a memory.
 21. An interrogationsystem, comprising: a radio frequency identification (RFID) objectconfigured to provide a reply code; and an interrogator, including: acorrelation subsystem configured to correlate a reference code at afirst clock rate with said reply code, correlate said reference code ata second clock rate with said reply code and provide correlation signalstherefrom, said correlation subsystem being configured to correlateemploying a Fast Fourier Transform and a convolution theorem, and adecision subsystem configured to verify a presence of said RFID objectas a function of said correlation signals.
 22. The interrogation systemas recited in claim 21 wherein said correlation subsystem includesmultiple correlators.
 23. The interrogation system as recited in claim21 wherein said reference code is a synthetically derived referencecode.
 24. The interrogation system as recited in claim 21 wherein saidcorrelation subsystem is configured to correlate in a time domainemploying an exclusive OR function.
 25. The interrogation system asrecited in claim 21 wherein said first clock rate is an expected clockrate and said second clock rate varies therefrom.
 26. The interrogationsystem as recited in claim 21 wherein said correlation subsystemincludes: a correlator configured to correlate at least two bits of saidreference code with at least two bits of said reply code to derivecorrelation triangles, a correlation threshold sense configured tocompare said correlation triangles to a threshold criteria to derivepulses to ascertain a peak of said correlation triangles, and a summerconfigured to average a plurality of pulses from said correlationthreshold sense to provide said correlation signals.
 27. Theinterrogation system as recited in claim 21 wherein said decisionsubsystem includes a threshold detector configured to compare saidcorrelation signals to a threshold.
 28. The interrogation system asrecited in claim 21 wherein said decision subsystem is configured toverify said presence of said RFID object by employing a statisticalanalysis on a result therefrom.
 29. The interrogation system as recitedin claim 21 wherein said reference code is configured to be providedduring an initialization stage of operation and said reply code isconfigured to be provided during a post-initialization stage ofoperation.
 30. The interrogation system as recited in claim 21 whereinsaid reference code is configured to be entered into a memory of saidinterrogator.